[jnigen] Use dollar signs for nested class name separator

pkgs/jnigen/CHANGELOG.md

@@ -1,5 +1,7 @@
 ## 0.13.0-wip
 
+- **Breaking Change**([#1516](https://github.com/dart-lang/native/issues/1516)):
+  Inner classes are now generated as `OuterClass$InnerClass`.
 - **Breaking Change**([#1644](https://github.com/dart-lang/native/issues/1644)):
   Generate null-safe Dart bindings for Java and Kotlin.
 - Fixed a potential name collision when generating in multi-file mode.

pkgs/jnigen/doc/java_differences.md

@@ -213,3 +213,27 @@ class $_Example extends JObject {
   void $_printMessage() { /* ... */ }
 }
 ```
+
+### Inner classes
+
+Java has the concept of inner classes, while Dart does not. Therefore, inner
+classes are generated as separate classes named using the name of their
+outer-class followed by a dollar sign (`$`) followed by their original name.
+
+For example:
+
+```java
+// Java
+public class Outer {
+  public class Inner {}
+}
+```
+
+will be turned into:
+
+```dart
+// Dart Bindings - Boilerplate omitted for clarity.
+class Outer extends JObject {}
+
+class Outer$Inner extends JObject {}
+```