[pigeon] Adds overrides for constructors and static members of ProxyApis (#9515)

Adds `PigeonOverrides` class that contains overrides for all constructors and static members of ProxyApis.

I changed it to have only a single `PigeonOverrides` instead of having a separate overrides class for each proxy api. This makes it easier to export the generated proxy api classes without needing to `hide/show` specific classes.

I manually verified this only breaks one line in `camera_android_camerax` and one line in `webview_flutter_android`.

Also TIL:

```
class MyClass {
  MyClass();          <--
                           These are considered the same.
  MyClass.new();      <--
}
```

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: bparrishMines

packages/pigeon/CHANGELOG.md

@@ -1,3 +1,9 @@
+## 26.0.0
+
+* **Breaking Change** [dart] Changes name of constructors used to create subclasses of ProxyApis to
+  `pigeon_**original_name**`.
+* [dart] Adds ProxyApi overrides classes to be used in Flutter unit tests.
+
 ## 25.5.0
 
 * [dart] Changes the default InstanceManager and its initialization to no longer make a message call

packages/pigeon/lib/src/ast.dart

@@ -240,34 +240,53 @@ class AstProxyApi extends Api {
   /// `implements`.
   Iterable<AstProxyApi> apisOfInterfaces() => _recursiveFindAllInterfaceApis();
 
-  /// All methods inherited from interfaces and the interfaces of interfaces.
-  Iterable<Method> flutterMethodsFromInterfaces() sync* {
+  /// Returns a record for each method inherited from an interface and its
+  /// corresponding ProxyApi.
+  Iterable<(Method, AstProxyApi)> flutterMethodsFromInterfacesWithApis() sync* {
     for (final AstProxyApi proxyApi in apisOfInterfaces()) {
-      yield* proxyApi.methods;
+      yield* proxyApi.methods.map((Method method) => (method, proxyApi));
     }
   }
 
-  /// A list of Flutter methods inherited from the ProxyApi that this ProxyApi
-  /// `extends`.
-  ///
-  /// This also recursively checks the ProxyApi that the super class `extends`
-  /// and so on.
+  /// Returns a record for each Flutter method inherited from [superClass].
   ///
   /// This also includes methods that super classes inherited from interfaces
   /// with `implements`.
-  Iterable<Method> flutterMethodsFromSuperClasses() sync* {
+  Iterable<(Method, AstProxyApi)>
+      flutterMethodsFromSuperClassesWithApis() sync* {
     for (final AstProxyApi proxyApi in allSuperClasses().toList().reversed) {
-      yield* proxyApi.flutterMethods;
+      yield* proxyApi.flutterMethods.map((Method method) => (method, proxyApi));
     }
     if (superClass != null) {
       final Set<AstProxyApi> interfaceApisFromSuperClasses =
           superClass!.associatedProxyApi!._recursiveFindAllInterfaceApis();
       for (final AstProxyApi proxyApi in interfaceApisFromSuperClasses) {
-        yield* proxyApi.methods;
+        yield* proxyApi.methods.map((Method method) => (method, proxyApi));
       }
     }
   }
 
+  /// All methods inherited from interfaces and the interfaces of interfaces.
+  Iterable<Method> flutterMethodsFromInterfaces() sync* {
+    yield* flutterMethodsFromInterfacesWithApis().map(
+      ((Method, AstProxyApi) method) => method.$1,
+    );
+  }
+
+  /// A list of Flutter methods inherited from the ProxyApi that this ProxyApi
+  /// `extends`.
+  ///
+  /// This also recursively checks the ProxyApi that the super class `extends`
+  /// and so on.
+  ///
+  /// This also includes methods that super classes inherited from interfaces
+  /// with `implements`.
+  Iterable<Method> flutterMethodsFromSuperClasses() sync* {
+    yield* flutterMethodsFromSuperClassesWithApis().map(
+      ((Method, AstProxyApi) method) => method.$1,
+    );
+  }
+
   /// Whether the API has a method that callbacks to Dart to add a new instance
   /// to the InstanceManager.
   ///