Add test for RenderProxyBoxMixin; clarify doc, resolve TODO (#117664)

* Add test for RenderProxyBoxMixin; clarify doc, resolve TODO

The TODO comment suggested this mixin would no longer be needed once
a Dart issue on inherited constructors was fixed:
  https://github.com/dart-lang/sdk/issues/31543
That issue is now long since fixed, so I went to go carry out the TODO.

But in doing so, I realized that the mixin's documentation was more
right than the TODO comment: even with that issue fixed, there is a
legitimate use case for this mixin, namely to reuse the implementation
of RenderProxyBox in a class that also inherits from some other base
class.  Moreover, searching GitHub I found an example of a library
that makes real use of that capability.

So I think the right resolution is to accept that this separation
is useful and delete the TODO.

Then, add a test with an extremely simplified sketch of that
real-world example.  In case someone in the future attempts to
simplify this mixin away, the test will point us at the use case
that would be broken by such a change.

Also remove the only in-tree use of the mixin, which was redundant;
and expand the mixin's documentation to advise about that case.

* Tweak formatting
Co-authored-by: Michael Goderbauer <goderbauer@google.com>

* Cut comments

---------
Co-authored-by: Michael Goderbauer <goderbauer@google.com>

packages/flutter/lib/src/rendering/proxy_box.dart

@@ -53,8 +53,10 @@ class RenderProxyBox extends RenderBox with RenderObjectWithChildMixin<RenderBox
 ///
 /// Use this mixin in situations where the proxying behavior
 /// of [RenderProxyBox] is desired but inheriting from [RenderProxyBox] is
-/// impractical (e.g. because you want to mix in other classes as well).
-// TODO(ianh): Remove this class once https://github.com/dart-lang/sdk/issues/31543 is fixed
+/// impractical (e.g. because you want to inherit from a different class).
+///
+/// If a class already inherits from [RenderProxyBox] and also uses this mixin,
+/// you can safely delete the use of the mixin.
 @optionalTypeArgs
 mixin RenderProxyBoxMixin<T extends RenderBox> on RenderBox, RenderObjectWithChildMixin<T> {
   @override
@@ -1095,7 +1097,7 @@ mixin RenderAnimatedOpacityMixin<T extends RenderObject> on RenderObjectWithChil
 ///
 /// This is a variant of [RenderOpacity] that uses an [Animation<double>] rather
 /// than a [double] to control the opacity.
-class RenderAnimatedOpacity extends RenderProxyBox with RenderProxyBoxMixin, RenderAnimatedOpacityMixin<RenderBox> {
+class RenderAnimatedOpacity extends RenderProxyBox with RenderAnimatedOpacityMixin<RenderBox> {
   /// Creates a partially transparent render object.
   ///
   /// The [opacity] argument must not be null.

packages/flutter/test/rendering/proxy_box_test.dart

@@ -963,6 +963,17 @@ void main() {
     expect(debugPaintClipOval(Clip.none), paintsExactlyCountTimes(#drawPath, 0));
     expect(debugPaintClipOval(Clip.none), paintsExactlyCountTimes(#drawParagraph, 0));
   });
+
+  test('RenderProxyBox behavior can be mixed in along with another base class', () {
+    final RenderFancyProxyBox fancyProxyBox = RenderFancyProxyBox(fancy: 6);
+    // Box has behavior from its base class:
+    expect(fancyProxyBox.fancyMethod(), 36);
+    // Box has behavior from RenderProxyBox:
+    expect(
+      fancyProxyBox.computeDryLayout(const BoxConstraints(minHeight: 8)),
+      const Size(0, 8),
+    );
+  });
 }
 
 class _TestRectClipper extends CustomClipper<Rect> {
@@ -1061,6 +1072,21 @@ class ConditionalRepaintBoundary extends RenderProxyBox {
 
 class TestOffsetLayerA extends OffsetLayer {}
 
+class RenderFancyBox extends RenderBox {
+  RenderFancyBox({required this.fancy}) : super();
+
+  late int fancy;
+
+  int fancyMethod() {
+    return fancy * fancy;
+  }
+}
+
+class RenderFancyProxyBox extends RenderFancyBox
+    with RenderObjectWithChildMixin<RenderBox>, RenderProxyBoxMixin<RenderBox> {
+  RenderFancyProxyBox({required super.fancy});
+}
+
 void expectAssertionError() {
   final FlutterErrorDetails errorDetails = TestRenderingFlutterBinding.instance.takeFlutterErrorDetails()!;
   final bool asserted = errorDetails.toString().contains('Failed assertion');