diff --git a/analysis_options.yaml b/analysis_options.yaml
index e536739953..f45f6c9689 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -8,16 +8,16 @@ analyzer:
 
 linter:
   rules:
-  - avoid_bool_literals_in_conditional_expressions
-  - avoid_classes_with_only_static_members
-  - avoid_private_typedef_functions
-  - avoid_returning_this
-  - avoid_unused_constructor_parameters
-  - cascade_invocations
-  - join_return_with_assignment
-  - missing_whitespace_between_adjacent_strings
-  - no_adjacent_strings_in_list
-  - no_runtimeType_toString
-  - prefer_const_declarations
-  - prefer_expression_function_bodies
-  - use_string_buffers
+    - avoid_bool_literals_in_conditional_expressions
+    - avoid_classes_with_only_static_members
+    - avoid_private_typedef_functions
+    - avoid_returning_this
+    - avoid_unused_constructor_parameters
+    - cascade_invocations
+    - join_return_with_assignment
+    - missing_whitespace_between_adjacent_strings
+    - no_adjacent_strings_in_list
+    - no_runtimeType_toString
+    - prefer_const_declarations
+    - prefer_expression_function_bodies
+    - use_string_buffers
diff --git a/pkgs/http/lib/http.dart b/pkgs/http/lib/http.dart
index 62004240c7..2e2a802c85 100644
--- a/pkgs/http/lib/http.dart
+++ b/pkgs/http/lib/http.dart
@@ -20,6 +20,7 @@ export 'src/base_response.dart';
 export 'src/byte_stream.dart';
 export 'src/client.dart' hide zoneClient;
 export 'src/exception.dart';
+export 'src/headers.dart';
 export 'src/multipart_file.dart';
 export 'src/multipart_request.dart';
 export 'src/request.dart';
diff --git a/pkgs/http/lib/retry.dart b/pkgs/http/lib/retry.dart
index dedba9a9e7..95e9e0aa08 100644
--- a/pkgs/http/lib/retry.dart
+++ b/pkgs/http/lib/retry.dart
@@ -133,10 +133,10 @@ final class RetryClient extends BaseClient {
 
   /// Returns a copy of [original] with the given [body].
   StreamedRequest _copyRequest(BaseRequest original, Stream<List<int>> body) {
-    final request = StreamedRequest(original.method, original.url)
+    final request = StreamedRequest(original.method, original.url,
+        headers: original.headers)
       ..contentLength = original.contentLength
       ..followRedirects = original.followRedirects
-      ..headers.addAll(original.headers)
       ..maxRedirects = original.maxRedirects
       ..persistentConnection = original.persistentConnection;
 
diff --git a/pkgs/http/lib/src/base_client.dart b/pkgs/http/lib/src/base_client.dart
index 48a7f92fe9..f99945b992 100644
--- a/pkgs/http/lib/src/base_client.dart
+++ b/pkgs/http/lib/src/base_client.dart
@@ -19,42 +19,42 @@ import 'streamed_response.dart';
 /// maybe [close], and then they get various convenience methods for free.
 abstract mixin class BaseClient implements Client {
   @override
-  Future<Response> head(Uri url, {Map<String, String>? headers}) =>
+  Future<Response> head(Uri url, {Object? headers}) =>
       _sendUnstreamed('HEAD', url, headers);
 
   @override
-  Future<Response> get(Uri url, {Map<String, String>? headers}) =>
+  Future<Response> get(Uri url, {Object? headers}) =>
       _sendUnstreamed('GET', url, headers);
 
   @override
   Future<Response> post(Uri url,
-          {Map<String, String>? headers, Object? body, Encoding? encoding}) =>
+          {Object? headers, Object? body, Encoding? encoding}) =>
       _sendUnstreamed('POST', url, headers, body, encoding);
 
   @override
   Future<Response> put(Uri url,
-          {Map<String, String>? headers, Object? body, Encoding? encoding}) =>
+          {Object? headers, Object? body, Encoding? encoding}) =>
       _sendUnstreamed('PUT', url, headers, body, encoding);
 
   @override
   Future<Response> patch(Uri url,
-          {Map<String, String>? headers, Object? body, Encoding? encoding}) =>
+          {Object? headers, Object? body, Encoding? encoding}) =>
       _sendUnstreamed('PATCH', url, headers, body, encoding);
 
   @override
   Future<Response> delete(Uri url,
-          {Map<String, String>? headers, Object? body, Encoding? encoding}) =>
+          {Object? headers, Object? body, Encoding? encoding}) =>
       _sendUnstreamed('DELETE', url, headers, body, encoding);
 
   @override
-  Future<String> read(Uri url, {Map<String, String>? headers}) async {
+  Future<String> read(Uri url, {Object? headers}) async {
     final response = await get(url, headers: headers);
     _checkResponseSuccess(url, response);
     return response.body;
   }
 
   @override
-  Future<Uint8List> readBytes(Uri url, {Map<String, String>? headers}) async {
+  Future<Uint8List> readBytes(Uri url, {Object? headers}) async {
     final response = await get(url, headers: headers);
     _checkResponseSuccess(url, response);
     return response.bodyBytes;
@@ -71,12 +71,10 @@ abstract mixin class BaseClient implements Client {
   Future<StreamedResponse> send(BaseRequest request);
 
   /// Sends a non-streaming [Request] and returns a non-streaming [Response].
-  Future<Response> _sendUnstreamed(
-      String method, Uri url, Map<String, String>? headers,
+  Future<Response> _sendUnstreamed(String method, Uri url, Object? headers,
       [Object? body, Encoding? encoding]) async {
-    var request = Request(method, url);
+    final request = Request(method, url, headers: headers);
 
-    if (headers != null) request.headers.addAll(headers);
     if (encoding != null) request.encoding = encoding;
     if (body != null) {
       if (body is String) {
diff --git a/pkgs/http/lib/src/base_request.dart b/pkgs/http/lib/src/base_request.dart
index 70a78695aa..6763e4b7de 100644
--- a/pkgs/http/lib/src/base_request.dart
+++ b/pkgs/http/lib/src/base_request.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'dart:collection';
-
 import 'package:meta/meta.dart';
 
 import '../http.dart' show ClientException, get;
@@ -11,6 +9,7 @@ import 'base_client.dart';
 import 'base_response.dart';
 import 'byte_stream.dart';
 import 'client.dart';
+import 'headers.dart';
 import 'streamed_response.dart';
 import 'utils.dart';
 
@@ -82,7 +81,7 @@ abstract class BaseRequest {
   // TODO(nweiz): automatically parse cookies from headers
 
   // TODO(nweiz): make this a HttpHeaders object
-  final Map<String, String> headers;
+  final Headers headers;
 
   /// Whether [finalize] has been called.
   bool get finalized => _finalized;
@@ -96,11 +95,9 @@ abstract class BaseRequest {
     return method;
   }
 
-  BaseRequest(String method, this.url)
+  BaseRequest(String method, this.url, {Object? headers})
       : method = _validateMethod(method),
-        headers = LinkedHashMap(
-            equals: (key1, key2) => key1.toLowerCase() == key2.toLowerCase(),
-            hashCode: (key) => key.toLowerCase().hashCode);
+        headers = Headers(headers);
 
   /// Finalizes the HTTP request in preparation for it being sent.
   ///
diff --git a/pkgs/http/lib/src/base_response.dart b/pkgs/http/lib/src/base_response.dart
index ed95f6cdb2..6962c445e3 100644
--- a/pkgs/http/lib/src/base_response.dart
+++ b/pkgs/http/lib/src/base_response.dart
@@ -4,6 +4,7 @@
 
 import 'base_client.dart';
 import 'base_request.dart';
+import 'headers.dart';
 
 /// The base class for HTTP responses.
 ///
@@ -47,20 +48,22 @@ abstract class BaseResponse {
   /// by a single space. Leading and trailing whitespace in header values are
   /// always removed.
   // TODO(nweiz): make this a HttpHeaders object.
-  final Map<String, String> headers;
+  final Headers headers;
 
   final bool isRedirect;
 
   /// Whether the server requested that a persistent connection be maintained.
   final bool persistentConnection;
 
-  BaseResponse(this.statusCode,
-      {this.contentLength,
-      this.request,
-      this.headers = const {},
-      this.isRedirect = false,
-      this.persistentConnection = true,
-      this.reasonPhrase}) {
+  BaseResponse(
+    this.statusCode, {
+    this.contentLength,
+    this.request,
+    Object? headers,
+    this.isRedirect = false,
+    this.persistentConnection = true,
+    this.reasonPhrase,
+  }) : headers = Headers(headers) {
     if (statusCode < 100) {
       throw ArgumentError('Invalid status code $statusCode.');
     } else if (contentLength != null && contentLength! < 0) {
diff --git a/pkgs/http/lib/src/browser_client.dart b/pkgs/http/lib/src/browser_client.dart
index 80db8b1291..821af09627 100644
--- a/pkgs/http/lib/src/browser_client.dart
+++ b/pkgs/http/lib/src/browser_client.dart
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:js_interop';
 
 import 'package:web/helpers.dart';
@@ -62,14 +63,22 @@ class BrowserClient extends BaseClient {
       ..open(request.method, '${request.url}', true)
       ..responseType = 'arraybuffer'
       ..withCredentials = withCredentials;
-    for (var header in request.headers.entries) {
-      xhr.setRequestHeader(header.key, header.value);
+
+    // Set the headers.
+    for (final (name, value) in request.headers.entries()) {
+      xhr.setRequestHeader(name, value);
+    }
+
+    // Sets cookies.
+    for (final cookie in request.headers.getSetCookie()) {
+      xhr.setRequestHeader('Set-Cookie', cookie);
     }
 
     var completer = Completer<StreamedResponse>();
 
     unawaited(xhr.onLoad.first.then((_) {
-      if (xhr.responseHeaders['content-length'] case final contentLengthHeader
+      if (xhr.responseHeaders.get('content-length')
+          case final contentLengthHeader
           when contentLengthHeader != null &&
               !_digitRegex.hasMatch(contentLengthHeader)) {
         completer.completeError(ClientException(
@@ -118,12 +127,11 @@ class BrowserClient extends BaseClient {
 }
 
 extension on XMLHttpRequest {
-  Map<String, String> get responseHeaders {
-    // from Closure's goog.net.Xhrio.getResponseHeaders.
-    var headers = <String, String>{};
-    var headersString = getAllResponseHeaders();
-    var headersList = headersString.split('\r\n');
-    for (var header in headersList) {
+  Headers get responseHeaders {
+    final headers = Headers();
+    final lines = const LineSplitter().convert(getAllResponseHeaders());
+
+    for (var header in lines) {
       if (header.isEmpty) {
         continue;
       }
@@ -132,14 +140,13 @@ extension on XMLHttpRequest {
       if (splitIdx == -1) {
         continue;
       }
+
       var key = header.substring(0, splitIdx).toLowerCase();
       var value = header.substring(splitIdx + 2);
-      if (headers.containsKey(key)) {
-        headers[key] = '${headers[key]}, $value';
-      } else {
-        headers[key] = value;
-      }
+
+      headers.append(key, value);
     }
+
     return headers;
   }
 }
diff --git a/pkgs/http/lib/src/headers.dart b/pkgs/http/lib/src/headers.dart
new file mode 100644
index 0000000000..9ffcf688eb
--- /dev/null
+++ b/pkgs/http/lib/src/headers.dart
@@ -0,0 +1,175 @@
+import 'dart:convert';
+
+/// This Fetch API interface allows you to perform various actions on HTTP
+/// request and response headers. These actions include retrieving, setting,
+/// adding to, and removing. A Headers object has an associated header list,
+/// which is initially empty and consists of zero or more name and value pairs.
+///  You can add to this using methods like append() (see Examples.) In all
+/// methods of this interface, header names are matched by case-insensitive
+/// byte sequence.
+///
+/// [MDN Reference](https://developer.mozilla.org/docs/Web/API/Headers)
+class Headers {
+  final List<(String, String)> _storage;
+
+  /// Internal constructor, to create a new instance of `Headers`.
+  const Headers._(this._storage);
+
+  /// The Headers() constructor creates a new Headers object.
+  ///
+  /// [MDN Reference](https://developer.mozilla.org/docs/Web/API/Headers/Headers)
+  factory Headers([Object? init]) => Headers._((init,).toStorage());
+
+  /// Appends a new value onto an existing header inside a Headers object, or
+  /// adds the header if it does not already exist.
+  ///
+  /// [MDN Reference](https://developer.mozilla.org/docs/Web/API/Headers/append)
+  void append(String name, String value) => _storage.add((name, value));
+
+  /// Deletes a header from a Headers object.
+  ///
+  /// [MDN Reference](https://developer.mozilla.org/docs/Web/API/Headers/delete)
+  void delete(String name) =>
+      _storage.removeWhere((element) => element.$1.equals(name));
+
+  /// Returns an iterator allowing to go through all key/value pairs contained
+  /// in this object.
+  ///
+  /// [MDN Reference](https://developer.mozilla.org/docs/Web/API/Headers/entries)
+  Iterable<(String, String)> entries() sync* {
+    for (final (name, value) in _storage) {
+      // https://fetch.spec.whatwg.org/#ref-for-forbidden-response-header-name%E2%91%A0
+      if (name.equals('set-cookie')) continue;
+
+      yield (name, value);
+    }
+  }
+
+  /// Executes a provided function once for each key/value pair in this Headers object.
+  ///
+  /// [MDN Reference](https://developer.mozilla.org/en-US/docs/Web/API/Headers/forEach)
+  void forEach(void Function(String value, String name, Headers parent) fn) =>
+      entries().forEach((element) => fn(element.$2, element.$1, this));
+
+  /// Returns a String sequence of all the values of a header within a Headers
+  /// object with a given name.
+  ///
+  /// [MDN Reference](https://developer.mozilla.org/docs/Web/API/Headers/get)
+  String? get(String name) => switch (_storage.valuesOf(name)) {
+        Iterable<String> values when values.isNotEmpty => values.join(', '),
+        _ => null,
+      };
+
+  /// Returns an array containing the values of all Set-Cookie headers
+  /// associated with a response.
+  ///
+  /// [MDN Reference](https://developer.mozilla.org/docs/Web/API/Headers/getSetCookie)
+  Iterable<String> getSetCookie() => _storage.valuesOf('Set-Cookie');
+
+  /// Returns a boolean stating whether a Headers object contains a certain
+  /// header.
+  ///
+  /// [MDN Reference](https://developer.mozilla.org/docs/Web/API/Headers/has)
+  bool has(String name) => _storage.any((element) => element.$1.equals(name));
+
+  /// Returns an iterator allowing you to go through all keys of the key/value
+  /// pairs contained in this object.
+  ///
+  /// [MDN Reference](https://developer.mozilla.org/docs/Web/API/Headers/keys)
+  Iterable<String> keys() => _storage.map((e) => e.$1).toSet();
+
+  /// Sets a new value for an existing header inside a Headers object, or adds
+  /// the header if it does not already exist.
+  ///
+  /// [MDN Reference](https://developer.mozilla.org/docs/Web/API/Headers/set)
+  void set(String name, String value) => this
+    ..delete(name)
+    ..append(name, value);
+
+  /// Returns an iterator allowing you to go through all values of the
+  /// key/value pairs contained in this object.
+  ///
+  /// [MDN Reference](https://developer.mozilla.org/docs/Web/API/Headers/values)
+  Iterable<String> values() => keys().map(get).whereType();
+}
+
+extension on String {
+  bool equals(String other) => other.toLowerCase() == toLowerCase();
+}
+
+extension on Iterable<(String, String)> {
+  Iterable<String> valuesOf(String name) =>
+      where((element) => element.$1.equals(name)).map((e) => e.$2);
+}
+
+extension on (Object?,) {
+  List<(String, String)> toStorage() => switch (this.$1) {
+        Headers value => value.toStorage(),
+        String value => value.toStorage(),
+        Iterable<String> value => value.toStorage(),
+        Iterable<(String, String)> value => value.toList(),
+        Iterable<Iterable<String>> value => value.toStorage(),
+        Map<String, String> value => value.toStorage(),
+        Map<String, Iterable<String>> value => value.toStorage(),
+        _ => [],
+      };
+}
+
+extension on Map<String, Iterable<String>> {
+  List<(String, String)> toStorage() => entries
+      .map((e) => e.value.map((value) => (e.key, value)))
+      .expand((e) => e)
+      .toList();
+}
+
+extension on Map<String, String> {
+  List<(String, String)> toStorage() =>
+      entries.map((e) => (e.key, e.value)).toList();
+}
+
+extension on Iterable<Iterable<String>> {
+  List<(String, String)> toStorage() {
+    final storage = <(String, String)>[];
+    for (final element in this) {
+      switch (element) {
+        case Iterable<String> value when value.length == 2:
+          storage.add((value.first, value.last));
+          break;
+        case Iterable<String> value when value.length == 1:
+          final pair = value.first.toHeadersPair();
+          if (pair != null) storage.add(pair);
+          break;
+        case Iterable<String> value when value.length > 2:
+          for (final element in value.skip(1)) {
+            storage.add((value.first, element));
+          }
+          break;
+      }
+    }
+
+    return storage;
+  }
+}
+
+extension on Iterable<String> {
+  List<(String, String)> toStorage() =>
+      map((e) => e.toHeadersPair()).whereType<(String, String)>().toList();
+}
+
+extension on Headers {
+  List<(String, String)> toStorage() => entries().toList();
+}
+
+extension on String {
+  /// Converts a string to a list of headers.
+  List<(String, String)> toStorage() =>
+      const LineSplitter().convert(this).toStorage();
+
+  /// Parses to a header pair.
+  (String, String)? toHeadersPair() {
+    final index = indexOf(':');
+    if (index == -1) return null;
+
+    return (substring(0, index), substring(index + 1).trim());
+  }
+}
diff --git a/pkgs/http/lib/src/io_client.dart b/pkgs/http/lib/src/io_client.dart
index db66b028c4..c0b45b69ad 100644
--- a/pkgs/http/lib/src/io_client.dart
+++ b/pkgs/http/lib/src/io_client.dart
@@ -102,10 +102,25 @@ class IOClient extends BaseClient {
         ..maxRedirects = request.maxRedirects
         ..contentLength = (request.contentLength ?? -1)
         ..persistentConnection = request.persistentConnection;
-      request.headers.forEach((name, value) {
-        ioRequest.headers.set(name, value);
+
+      final defaultUserAgent = ioRequest.headers.value('user-agent');
+      ioRequest.headers.removeAll('user-agent');
+
+      // Set the headers.
+      request.headers.forEach((value, name, _) {
+        ioRequest.headers.add(name, value);
       });
 
+      if (ioRequest.headers.value('user-agent') == null &&
+          defaultUserAgent != null) {
+        ioRequest.headers.set('user-agent', defaultUserAgent);
+      }
+
+      // Set cookies.
+      for (final cookie in request.headers.getSetCookie()) {
+        ioRequest.headers.add('set-cookie', cookie);
+      }
+
       var response = await stream.pipe(ioRequest) as HttpClientResponse;
 
       var headers = <String, String>{};
diff --git a/pkgs/http/lib/src/mock_client.dart b/pkgs/http/lib/src/mock_client.dart
index 52f108a577..b1a6a49166 100644
--- a/pkgs/http/lib/src/mock_client.dart
+++ b/pkgs/http/lib/src/mock_client.dart
@@ -7,6 +7,7 @@ import 'dart:convert';
 import 'base_client.dart';
 import 'base_request.dart';
 import 'byte_stream.dart';
+import 'headers.dart';
 import 'request.dart';
 import 'response.dart';
 import 'streamed_request.dart';
@@ -100,3 +101,15 @@ typedef MockClientStreamHandler = Future<StreamedResponse> Function(
 ///
 /// Note that [request] will be finalized.
 typedef MockClientHandler = Future<Response> Function(Request request);
+
+extension on Headers {
+  /// Appends other headers to this.
+  void addAll(Headers other) {
+    other.forEach((value, name, _) => append(name, value));
+
+    // Set cookies.
+    for (final cookie in other.getSetCookie()) {
+      append('set-cookie', cookie);
+    }
+  }
+}
diff --git a/pkgs/http/lib/src/multipart_request.dart b/pkgs/http/lib/src/multipart_request.dart
index 79525421fb..d0219c4c3b 100644
--- a/pkgs/http/lib/src/multipart_request.dart
+++ b/pkgs/http/lib/src/multipart_request.dart
@@ -87,7 +87,8 @@ class MultipartRequest extends BaseRequest {
   ByteStream finalize() {
     // TODO: freeze fields and files
     final boundary = _boundaryString();
-    headers['content-type'] = 'multipart/form-data; boundary=$boundary';
+    headers.set('content-type', 'multipart/form-data; boundary=$boundary');
+
     super.finalize();
     return ByteStream(_finalize(boundary));
   }
diff --git a/pkgs/http/lib/src/request.dart b/pkgs/http/lib/src/request.dart
index c15e55169d..c0b822ceb5 100644
--- a/pkgs/http/lib/src/request.dart
+++ b/pkgs/http/lib/src/request.dart
@@ -149,7 +149,7 @@ class Request extends BaseRequest {
     body = mapToQuery(fields, encoding: encoding);
   }
 
-  Request(super.method, super.url)
+  Request(super.method, super.url, {super.headers})
       : _defaultEncoding = utf8,
         _bodyBytes = Uint8List(0);
 
@@ -163,17 +163,17 @@ class Request extends BaseRequest {
 
   /// The `Content-Type` header of the request (if it exists) as a [MediaType].
   MediaType? get _contentType {
-    var contentType = headers['content-type'];
+    var contentType = headers.get('content-type');
     if (contentType == null) return null;
     return MediaType.parse(contentType);
   }
 
   set _contentType(MediaType? value) {
     if (value == null) {
-      headers.remove('content-type');
-    } else {
-      headers['content-type'] = value.toString();
+      return headers.delete('content-type');
     }
+
+    headers.set('content-type', value.toString());
   }
 
   /// Throw an error if this request has been finalized.
diff --git a/pkgs/http/lib/src/response.dart b/pkgs/http/lib/src/response.dart
index 1ba7c466cf..cac141de99 100644
--- a/pkgs/http/lib/src/response.dart
+++ b/pkgs/http/lib/src/response.dart
@@ -9,6 +9,7 @@ import 'package:http_parser/http_parser.dart';
 
 import 'base_request.dart';
 import 'base_response.dart';
+import 'headers.dart';
 import 'streamed_response.dart';
 import 'utils.dart';
 
@@ -30,7 +31,7 @@ class Response extends BaseResponse {
   /// Creates a new HTTP response with a string body.
   Response(String body, int statusCode,
       {BaseRequest? request,
-      Map<String, String> headers = const {},
+      Headers? headers,
       bool isRedirect = false,
       bool persistentConnection = true,
       String? reasonPhrase})
@@ -68,14 +69,14 @@ class Response extends BaseResponse {
 ///
 /// Defaults to [latin1] if the headers don't specify a charset or if that
 /// charset is unknown.
-Encoding _encodingForHeaders(Map<String, String> headers) =>
+Encoding _encodingForHeaders(Headers? headers) =>
     encodingForCharset(_contentTypeForHeaders(headers).parameters['charset']);
 
 /// Returns the [MediaType] object for the given headers's content-type.
 ///
 /// Defaults to `application/octet-stream`.
-MediaType _contentTypeForHeaders(Map<String, String> headers) {
-  var contentType = headers['content-type'];
+MediaType _contentTypeForHeaders(Headers? headers) {
+  var contentType = headers?.get('content-type');
   if (contentType != null) return MediaType.parse(contentType);
   return MediaType('application', 'octet-stream');
 }
diff --git a/pkgs/http/lib/src/streamed_request.dart b/pkgs/http/lib/src/streamed_request.dart
index d10386e263..bb7e3ac998 100644
--- a/pkgs/http/lib/src/streamed_request.dart
+++ b/pkgs/http/lib/src/streamed_request.dart
@@ -42,7 +42,7 @@ class StreamedRequest extends BaseRequest {
   final StreamController<List<int>> _controller;
 
   /// Creates a new streaming request.
-  StreamedRequest(super.method, super.url)
+  StreamedRequest(super.method, super.url, {super.headers})
       : _controller = StreamController<List<int>>(sync: true);
 
   /// Freezes all mutable fields and returns a single-subscription [ByteStream]
diff --git a/pkgs/http/test/http_retry_test.dart b/pkgs/http/test/http_retry_test.dart
index da51154c4a..fa508a717c 100644
--- a/pkgs/http/test/http_retry_test.dart
+++ b/pkgs/http/test/http_retry_test.dart
@@ -53,13 +53,13 @@ void main() {
         MockClient(expectAsync1((request) async {
           count++;
           return Response('', 503,
-              headers: {'retry': count < 2 ? 'true' : 'false'});
+              headers: Headers({'retry': count < 2 ? 'true' : 'false'}));
         }, count: 2)),
-        when: (response) => response.headers['retry'] == 'true',
+        when: (response) => response.headers.get('retry') == 'true',
         delay: (_) => Duration.zero);
 
     final response = await client.get(Uri.http('example.org', ''));
-    expect(response.headers, containsPair('retry', 'false'));
+    expect(response.headers.get('retry'), equals('false'));
     expect(response.statusCode, equals(503));
   });
 
@@ -204,7 +204,9 @@ void main() {
         MockClient(expectAsync1((request) async {
           expect(request.contentLength, equals(5));
           expect(request.followRedirects, isFalse);
-          expect(request.headers, containsPair('foo', 'bar'));
+          // // expect(request.headers, containsPair('foo', 'bar'));
+          expect(request.headers.keys(), contains('foo'));
+          expect(request.headers.values(), contains('bar'));
           expect(request.maxRedirects, equals(12));
           expect(request.method, equals('POST'));
           expect(request.persistentConnection, isFalse);
@@ -217,7 +219,8 @@ void main() {
     final request = Request('POST', Uri.http('example.org', ''))
       ..body = 'hello'
       ..followRedirects = false
-      ..headers['foo'] = 'bar'
+      // ..headers['foo'] = 'bar'
+      ..headers.set('foo', 'bar')
       ..maxRedirects = 12
       ..persistentConnection = false;
 
@@ -228,7 +231,7 @@ void main() {
   test('async when, whenError and onRetry', () async {
     final client = RetryClient(
       MockClient(expectAsync1(
-          (request) async => request.headers['Authorization'] != null
+          (request) async => request.headers.get('Authorization') != null
               ? Response('', 200)
               : Response('', 401),
           count: 2)),
@@ -245,7 +248,8 @@ void main() {
       onRetry: (request, response, retryCount) async {
         expect(response?.statusCode, equals(401));
         await Future<void>.delayed(const Duration(milliseconds: 500));
-        request.headers['Authorization'] = 'Bearer TOKEN';
+        // request.headers['Authorization'] = 'Bearer TOKEN';
+        request.headers.set('Authorization', 'Bearer TOKEN');
       },
     );
 
diff --git a/pkgs/http/test/io/client_test.dart b/pkgs/http/test/io/client_test.dart
index fd426a8b3f..9e861164a6 100644
--- a/pkgs/http/test/io/client_test.dart
+++ b/pkgs/http/test/io/client_test.dart
@@ -38,9 +38,12 @@ void main() {
   test('#send a StreamedRequest', () async {
     var client = http.Client();
     var request = http.StreamedRequest('POST', serverUrl)
-      ..headers[HttpHeaders.contentTypeHeader] =
-          'application/json; charset=utf-8'
-      ..headers[HttpHeaders.userAgentHeader] = 'Dart';
+      // ..headers[HttpHeaders.contentTypeHeader] =
+      //     'application/json; charset=utf-8'
+      // ..headers[HttpHeaders.userAgentHeader] = 'Dart';
+      ..headers
+          .set(HttpHeaders.contentTypeHeader, 'application/json; charset=utf-8')
+      ..headers.set(HttpHeaders.userAgentHeader, 'Dart');
 
     var responseFuture = client.send(request);
     request.sink.add('{"hello": "world"}'.codeUnits);
@@ -50,7 +53,7 @@ void main() {
 
     expect(response.request, equals(request));
     expect(response.statusCode, equals(200));
-    expect(response.headers['single'], equals('value'));
+    expect(response.headers.get('single'), equals('value'));
     // dart:io internally normalizes outgoing headers so that they never
     // have multiple headers with the same name, so there's no way to test
     // whether we handle that case correctly.
@@ -76,9 +79,12 @@ void main() {
     var ioClient = HttpClient();
     var client = http_io.IOClient(ioClient);
     var request = http.StreamedRequest('POST', serverUrl)
-      ..headers[HttpHeaders.contentTypeHeader] =
-          'application/json; charset=utf-8'
-      ..headers[HttpHeaders.userAgentHeader] = 'Dart';
+      // ..headers[HttpHeaders.contentTypeHeader] =
+      //     'application/json; charset=utf-8'
+      ..headers
+          .set(HttpHeaders.contentTypeHeader, 'application/json; charset=utf-8')
+      // ..headers[HttpHeaders.userAgentHeader] = 'Dart';
+      ..headers.set(HttpHeaders.userAgentHeader, 'Dart');
 
     var responseFuture = client.send(request);
     request.sink.add('{"hello": "world"}'.codeUnits);
@@ -88,7 +94,7 @@ void main() {
 
     expect(response.request, equals(request));
     expect(response.statusCode, equals(200));
-    expect(response.headers['single'], equals('value'));
+    expect(response.headers.get('single'), equals('value'));
     // dart:io internally normalizes outgoing headers so that they never
     // have multiple headers with the same name, so there's no way to test
     // whether we handle that case correctly.
@@ -114,8 +120,8 @@ void main() {
     var client = http.Client();
     var url = Uri.http('http.invalid', '');
     var request = http.StreamedRequest('POST', url);
-    request.headers[HttpHeaders.contentTypeHeader] =
-        'application/json; charset=utf-8';
+    request.headers
+        .set(HttpHeaders.contentTypeHeader, 'application/json; charset=utf-8');
 
     expect(
         client.send(request),
diff --git a/pkgs/http/test/io/request_test.dart b/pkgs/http/test/io/request_test.dart
index ac6b44c3fd..fd23ff11ca 100644
--- a/pkgs/http/test/io/request_test.dart
+++ b/pkgs/http/test/io/request_test.dart
@@ -19,7 +19,8 @@ void main() {
   test('send happy case', () async {
     final request = http.Request('GET', serverUrl)
       ..body = 'hello'
-      ..headers['User-Agent'] = 'Dart';
+      // ..headers['User-Agent'] = 'Dart';
+      ..headers.set('User-Agent', 'Dart');
 
     final response = await request.send();
 
diff --git a/pkgs/http/test/mock_client_test.dart b/pkgs/http/test/mock_client_test.dart
index 625285cb33..c9dc0ea5fc 100644
--- a/pkgs/http/test/mock_client_test.dart
+++ b/pkgs/http/test/mock_client_test.dart
@@ -5,6 +5,7 @@
 import 'dart:convert';
 
 import 'package:http/http.dart' as http;
+import 'package:http/src/headers.dart';
 import 'package:http/src/request.dart';
 import 'package:http/testing.dart';
 import 'package:test/test.dart';
@@ -15,7 +16,8 @@ void main() {
   test('handles a request', () async {
     var client = MockClient((request) async => http.Response(
         json.encode(request.bodyFields), 200,
-        request: request, headers: {'content-type': 'application/json'}));
+        request: request,
+        headers: Headers({'content-type': 'application/json'})));
 
     var response = await client.post(Uri.http('example.com', '/foo'),
         body: {'field1': 'value1', 'field2': 'value2'});
@@ -52,7 +54,7 @@ void main() {
         [137, 80, 78, 71, 13, 10, 26, 10] // PNG header
         );
     expect(response.request, null);
-    expect(response.headers, containsPair('content-type', 'image/png'));
+    expect(response.headers.get('content-type'), contains('image/png'));
   });
 
   test('pngResponse with request', () {
@@ -63,6 +65,6 @@ void main() {
         [137, 80, 78, 71, 13, 10, 26, 10] // PNG header
         );
     expect(response.request, request);
-    expect(response.headers, containsPair('content-type', 'image/png'));
+    expect(response.headers.get('content-type'), contains('image/png'));
   });
 }
diff --git a/pkgs/http/test/request_test.dart b/pkgs/http/test/request_test.dart
index 59cb0988c5..4950e90f8a 100644
--- a/pkgs/http/test/request_test.dart
+++ b/pkgs/http/test/request_test.dart
@@ -43,8 +43,8 @@ void main() {
     });
 
     test('is based on the content-type charset if it exists', () {
-      var request = http.Request('POST', dummyUrl);
-      request.headers['Content-Type'] = 'text/plain; charset=iso-8859-1';
+      var request = http.Request('POST', dummyUrl,
+          headers: {'Content-Type': 'text/plain; charset=iso-8859-1'});
       expect(request.encoding.name, equals(latin1.name));
     });
 
@@ -52,17 +52,17 @@ void main() {
         () {
       var request = http.Request('POST', dummyUrl)
         ..encoding = latin1
-        ..headers['Content-Type'] = 'text/plain; charset=utf-8';
+        ..headers.set('Content-Type', 'text/plain; charset=utf-8');
       expect(request.encoding.name, equals(utf8.name));
 
-      request.headers.remove('Content-Type');
+      request.headers.delete('Content-Type');
       expect(request.encoding.name, equals(latin1.name));
     });
 
     test('throws an error if the content-type charset is unknown', () {
       var request = http.Request('POST', dummyUrl);
-      request.headers['Content-Type'] =
-          'text/plain; charset=not-a-real-charset';
+      request.headers
+          .set('Content-Type', 'text/plain; charset=not-a-real-charset');
       expect(() => request.encoding, throwsFormatException);
     });
   });
@@ -125,19 +125,21 @@ void main() {
 
     test("can't be read with the wrong content-type", () {
       var request = http.Request('POST', dummyUrl);
-      request.headers['Content-Type'] = 'text/plain';
+      request.headers.set('Content-Type', 'text/plain');
       expect(() => request.bodyFields, throwsStateError);
     });
 
     test("can't be set with the wrong content-type", () {
       var request = http.Request('POST', dummyUrl);
-      request.headers['Content-Type'] = 'text/plain';
+      // request.headers['Content-Type'] = 'text/plain';
+      request.headers.set('Content-Type', 'text/plain');
       expect(() => request.bodyFields = {}, throwsStateError);
     });
 
     test('defaults to empty', () {
       var request = http.Request('POST', dummyUrl);
-      request.headers['Content-Type'] = 'application/x-www-form-urlencoded';
+      // request.headers['Content-Type'] = 'application/x-www-form-urlencoded';
+      request.headers.set('Content-Type', 'application/x-www-form-urlencoded');
       expect(request.bodyFields, isEmpty);
     });
 
@@ -149,7 +151,8 @@ void main() {
 
     test('changes when body changes', () {
       var request = http.Request('POST', dummyUrl);
-      request.headers['Content-Type'] = 'application/x-www-form-urlencoded';
+      // request.headers['Content-Type'] = 'application/x-www-form-urlencoded';
+      request.headers.set('Content-Type', 'application/x-www-form-urlencoded');
       request.body = 'key%201=value&key+2=other%2bvalue';
       expect(request.bodyFields,
           equals({'key 1': 'value', 'key 2': 'other+value'}));
@@ -157,7 +160,8 @@ void main() {
 
     test('is encoded according to the given encoding', () {
       var request = http.Request('POST', dummyUrl)
-        ..headers['Content-Type'] = 'application/x-www-form-urlencoded'
+        // ..headers['Content-Type'] = 'application/x-www-form-urlencoded'
+        ..headers.set('Content-Type', 'application/x-www-form-urlencoded')
         ..encoding = latin1
         ..bodyFields = {'föø': 'bãr'};
       expect(request.body, equals('f%F6%F8=b%E3r'));
@@ -165,7 +169,8 @@ void main() {
 
     test('is decoded according to the given encoding', () {
       var request = http.Request('POST', dummyUrl)
-        ..headers['Content-Type'] = 'application/x-www-form-urlencoded'
+        // ..headers['Content-Type'] = 'application/x-www-form-urlencoded'
+        ..headers.set('Content-Type', 'application/x-www-form-urlencoded')
         ..encoding = latin1
         ..body = 'f%F6%F8=b%E3r';
       expect(request.bodyFields, equals({'föø': 'bãr'}));
@@ -175,18 +180,19 @@ void main() {
   group('content-type header', () {
     test('defaults to empty', () {
       var request = http.Request('POST', dummyUrl);
-      expect(request.headers['Content-Type'], isNull);
+      expect(request.headers.get('Content-Type'), isNull);
     });
 
     test('defaults to empty if only encoding is set', () {
       var request = http.Request('POST', dummyUrl)..encoding = latin1;
-      expect(request.headers['Content-Type'], isNull);
+      // expect(request.headers['Content-Type'], isNull);
+      expect(request.headers.get('Content-Type'), isNull);
     });
 
     test('name is case insensitive', () {
       var request = http.Request('POST', dummyUrl);
-      request.headers['CoNtEnT-tYpE'] = 'application/json';
-      expect(request.headers, containsPair('content-type', 'application/json'));
+      request.headers.set('CoNtEnT-tYpE', 'application/json');
+      expect(request.headers.get('content-type'), equals('application/json'));
     });
 
     test(
@@ -194,7 +200,8 @@ void main() {
         'bodyFields is set', () {
       var request = http.Request('POST', dummyUrl)
         ..bodyFields = {'hello': 'world'};
-      expect(request.headers['Content-Type'],
+      // expect(request.headers['Content-Type'],
+      expect(request.headers.get('Content-Type'),
           equals('application/x-www-form-urlencoded; charset=utf-8'));
     });
 
@@ -204,7 +211,8 @@ void main() {
       var request = http.Request('POST', dummyUrl)
         ..encoding = latin1
         ..bodyFields = {'hello': 'world'};
-      expect(request.headers['Content-Type'],
+      // expect(request.headers['Content-Type'],
+      expect(request.headers.get('Content-Type'),
           equals('application/x-www-form-urlencoded; charset=iso-8859-1'));
     });
 
@@ -214,48 +222,60 @@ void main() {
       var request = http.Request('POST', dummyUrl)
         ..encoding = latin1
         ..body = 'hello, world';
-      expect(request.headers['Content-Type'],
+      // expect(request.headers['Content-Type'],
+      expect(request.headers.get('Content-Type'),
           equals('text/plain; charset=iso-8859-1'));
     });
 
     test('is modified to include utf-8 if body is set', () {
       var request = http.Request('POST', dummyUrl);
-      request.headers['Content-Type'] = 'application/json';
+      // request.headers['Content-Type'] = 'application/json';
+      request.headers.set('Content-Type', 'application/json');
       request.body = '{"hello": "world"}';
-      expect(request.headers['Content-Type'],
+      // expect(request.headers['Content-Type'],
+      expect(request.headers.get('Content-Type'),
           equals('application/json; charset=utf-8'));
     });
 
     test('is modified to include the given encoding if encoding is set', () {
       var request = http.Request('POST', dummyUrl);
-      request.headers['Content-Type'] = 'application/json';
+      // request.headers['Content-Type'] = 'application/json';
+      request.headers.set('Content-Type', 'application/json');
       request.encoding = latin1;
-      expect(request.headers['Content-Type'],
+      // expect(request.headers['Content-Type'],
+      expect(request.headers.get('Content-Type'),
           equals('application/json; charset=iso-8859-1'));
     });
 
     test('has its charset overridden by an explicit encoding', () {
       var request = http.Request('POST', dummyUrl);
-      request.headers['Content-Type'] = 'application/json; charset=utf-8';
+      // request.headers['Content-Type'] = 'application/json; charset=utf-8';
+      request.headers.set('Content-Type', 'application/json; charset=utf-8');
       request.encoding = latin1;
-      expect(request.headers['Content-Type'],
+      // expect(request.headers['Content-Type'],
+      expect(request.headers.get('Content-Type'),
           equals('application/json; charset=iso-8859-1'));
     });
 
     test("doesn't have its charset overridden by setting bodyFields", () {
       var request = http.Request('POST', dummyUrl);
-      request.headers['Content-Type'] =
-          'application/x-www-form-urlencoded; charset=iso-8859-1';
+
+      request.headers.set('Content-Type',
+          'application/x-www-form-urlencoded; charset=iso-8859-1');
       request.bodyFields = {'hello': 'world'};
-      expect(request.headers['Content-Type'],
+      // expect(request.headers['Content-Type'],
+      expect(request.headers.get('Content-Type'),
           equals('application/x-www-form-urlencoded; charset=iso-8859-1'));
     });
 
     test("doesn't have its charset overridden by setting body", () {
       var request = http.Request('POST', dummyUrl);
-      request.headers['Content-Type'] = 'application/json; charset=iso-8859-1';
+      // request.headers['Content-Type'] = 'application/json; charset=iso-8859-1';
+      request.headers
+          .set('Content-Type', 'application/json; charset=iso-8859-1');
       request.body = '{"hello": "world"}';
-      expect(request.headers['Content-Type'],
+      // expect(request.headers['Content-Type'],
+      expect(request.headers.get('Content-Type'),
           equals('application/json; charset=iso-8859-1'));
     });
   });
diff --git a/pkgs/http/test/response_test.dart b/pkgs/http/test/response_test.dart
index 38061c1ef4..bd80064203 100644
--- a/pkgs/http/test/response_test.dart
+++ b/pkgs/http/test/response_test.dart
@@ -5,6 +5,7 @@
 import 'dart:async';
 
 import 'package:http/http.dart' as http;
+import 'package:http/src/headers.dart';
 import 'package:test/test.dart';
 
 void main() {
@@ -24,7 +25,7 @@ void main() {
 
     test('respects the inferred encoding', () {
       var response = http.Response('föøbãr', 200,
-          headers: {'content-type': 'text/plain; charset=iso-8859-1'});
+          headers: Headers({'content-type': 'text/plain; charset=iso-8859-1'}));
       expect(response.bodyBytes, equals([102, 246, 248, 98, 227, 114]));
     });
   });
diff --git a/pkgs/http/test/utils.dart b/pkgs/http/test/utils.dart
index d4c319f73f..38759d276e 100644
--- a/pkgs/http/test/utils.dart
+++ b/pkgs/http/test/utils.dart
@@ -91,7 +91,7 @@ class _BodyMatches extends Matcher {
   Future<void> _checks(http.MultipartRequest item) async {
     var bodyBytes = await item.finalize().toBytes();
     var body = utf8.decode(bodyBytes);
-    var contentType = MediaType.parse(item.headers['content-type']!);
+    var contentType = MediaType.parse(item.headers.get('content-type')!);
     var boundary = contentType.parameters['boundary']!;
     var expected = cleanUpLiteral(_pattern)
         .replaceAll('\n', '\r\n')