Add meta tag @optionalTypeArgs to the mixin classes where we don't expect a...

Add meta tag @optionalTypeArgs to the mixin classes where we don't expect a generic type to be specified at the points where the mixin is applied. This suppresses the analyzer warnings.

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

@@ -51,7 +51,8 @@ class RenderProxyBox extends RenderBox with RenderObjectWithChildMixin<RenderBox
 /// This class can be used as a mixin for 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/15101 is fixed
+/// TODO(ianh): Remove this class once https://github.com/dart-lang/sdk/issues/15101 is fixed
+@optionalTypeArgs 
 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.

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

@@ -299,6 +299,7 @@ class KeepAliveHandle extends ChangeNotifier {
 ///
 ///  * [AutomaticKeepAlive], which listens to messages from this mixin.
 ///  * [KeepAliveNotification], the notifications sent by this mixin.
+@optionalTypeArgs
 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.

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

@@ -73,6 +73,7 @@ class TickerMode extends InheritedWidget {
 /// 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.
+@optionalTypeArgs
 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.
@@ -155,6 +156,7 @@ abstract class SingleTickerProviderStateMixin<T extends StatefulWidget> extends
 /// 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.
+@optionalTypeArgs
 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.