refactor(ngrouter): fix all static warnings (#55)

ngrouter/analysis_options.yaml

@@ -3,7 +3,4 @@ include: ../analysis_options.yaml
 analyzer:
   errors:
     # False positives related to missing .template.dart files
-    argument_type_not_assignable: ignore
-    invalid_assignment: ignore
-    return_of_invalid_type: ignore
-    return_of_invalid_type_from_closure: ignore
+    implementation_imports: ignore

ngrouter/test/revert_popstate_test.dart

@@ -1,4 +1,3 @@
-import 'dart:async' show Completer;
 import 'dart:html' show window;
 
 import 'package:test/test.dart';
@@ -47,26 +46,26 @@ void main() {
     // resolved. In order to be sure we're testing the correct state, we listen
     // for the next `popstate` event and use a completer to signal that it has
     // occured.
-    var nextPopState = Completer<void>()..complete(window.onPopState.first);
+    var nextPopState = window.onPopState.first;
     // Prevent navigation on back button.
     await testFixture.update((_) {
       routerHook.canLeave = false;
       location.back();
     });
     // In rare cases, not waiting for this `popstate` event causes the
     // subsequent code to execute first.
-    await nextPopState.future;
+    await nextPopState;
 
     // Location should not have changed.
     expect(location.path(), '/c');
 
-    nextPopState = Completer<void>()..complete(window.onPopState.first);
+    nextPopState = window.onPopState.first;
     // Allow navigation on back button.
     await testFixture.update((_) {
       routerHook.canLeave = true;
       location.back();
     });
-    await nextPopState.future;
+    await nextPopState;
 
     // Location should now be the correct previous history location.
     expect(location.path(), '/b');
@@ -79,7 +78,7 @@ const testModule = Module(
 );
 
 @GenerateInjector.fromModules([testModule])
-final createInjector = ng.createInjector$Injector;
+final InjectorFactory createInjector = ng.createInjector$Injector;
 
 @Component(
   selector: 'test',