Fixes in flutter framework code to address the mixin issue raised
in https://github.com/dart-lang/sdk/issues/31984

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

diff --git a/packages/flutter/lib/src/rendering/proxy_box.dart b/packages/flutter/lib/src/rendering/proxy_box.dart
index 67f68d3..3b80195 100644
--- a/packages/flutter/lib/src/rendering/proxy_box.dart
+++ b/packages/flutter/lib/src/rendering/proxy_box.dart
@@ -52,7 +52,7 @@
 /// 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/15101 is fixed
-abstract class RenderProxyBoxMixin extends RenderBox with RenderObjectWithChildMixin<RenderBox> {
+abstract class RenderProxyBoxMixin<T extends RenderBox> extends RenderBox with RenderObjectWithChildMixin<T> {
   // This class is intended to be used as a mixin, and should not be
   // extended directly.
   factory RenderProxyBoxMixin._() => null;

packages/flutter/lib/src/widgets/automatic_keep_alive.dart

diff --git a/packages/flutter/lib/src/widgets/automatic_keep_alive.dart b/packages/flutter/lib/src/widgets/automatic_keep_alive.dart
index 3f04dc2..ad264d8 100644
--- a/packages/flutter/lib/src/widgets/automatic_keep_alive.dart
+++ b/packages/flutter/lib/src/widgets/automatic_keep_alive.dart
@@ -299,7 +299,7 @@
 ///
 ///  * [AutomaticKeepAlive], which listens to messages from this mixin.
 ///  * [KeepAliveNotification], the notifications sent by this mixin.
-abstract class AutomaticKeepAliveClientMixin extends State<StatefulWidget> {
+abstract class AutomaticKeepAliveClientMixin<T extends StatefulWidget> extends State<T> {
   // This class is intended to be used as a mixin, and should not be
   // extended directly.
   factory AutomaticKeepAliveClientMixin._() => null;
@@ -358,4 +358,4 @@
       _ensureKeepAlive();
     return null;
   }
-}
\ No newline at end of file
+}

packages/flutter/lib/src/widgets/ticker_provider.dart

diff --git a/packages/flutter/lib/src/widgets/ticker_provider.dart b/packages/flutter/lib/src/widgets/ticker_provider.dart
index 4bb2e93..5449e8f 100644
--- a/packages/flutter/lib/src/widgets/ticker_provider.dart
+++ b/packages/flutter/lib/src/widgets/ticker_provider.dart
@@ -73,7 +73,7 @@
 /// This mixin only supports vending a single ticker. If you might have multiple
 /// [AnimationController] objects over the lifetime of the [State], use a full
 /// [TickerProviderStateMixin] instead.
-abstract class SingleTickerProviderStateMixin extends State<dynamic> implements TickerProvider { // ignore: TYPE_ARGUMENT_NOT_MATCHING_BOUNDS, https://github.com/dart-lang/sdk/issues/25232
+abstract class SingleTickerProviderStateMixin<T extends StatefulWidget> extends State<T> implements TickerProvider { // ignore: TYPE_ARGUMENT_NOT_MATCHING_BOUNDS, https://github.com/dart-lang/sdk/issues/25232
   // This class is intended to be used as a mixin, and should not be
   // extended directly.
   factory SingleTickerProviderStateMixin._() => null;
@@ -155,7 +155,7 @@
 /// If you only have a single [Ticker] (for example only a single
 /// [AnimationController]) for the lifetime of your [State], then using a
 /// [SingleTickerProviderStateMixin] is more efficient. This is the common case.
-abstract class TickerProviderStateMixin extends State<dynamic> implements TickerProvider { // ignore: TYPE_ARGUMENT_NOT_MATCHING_BOUNDS, https://github.com/dart-lang/sdk/issues/25232
+abstract class TickerProviderStateMixin<T extends StatefulWidget> extends State<T> implements TickerProvider { // ignore: TYPE_ARGUMENT_NOT_MATCHING_BOUNDS, https://github.com/dart-lang/sdk/issues/25232
   // This class is intended to be used as a mixin, and should not be
   // extended directly.
   factory TickerProviderStateMixin._() => null;
@@ -231,7 +231,7 @@
 class _WidgetTicker extends Ticker {
   _WidgetTicker(TickerCallback onTick, this._creator, { String debugLabel }) : super(onTick, debugLabel: debugLabel);
 
-  final TickerProviderStateMixin _creator;
+  final TickerProviderStateMixin<StatefulWidget> _creator;
 
   @override
   void dispose() {