address review comment.

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 0248589..c7927fb 100644
--- a/packages/flutter/lib/src/widgets/ticker_provider.dart
+++ b/packages/flutter/lib/src/widgets/ticker_provider.dart
@@ -233,7 +233,7 @@
 class _WidgetTicker extends Ticker {
   _WidgetTicker(TickerCallback onTick, this._creator, { String debugLabel }) : super(onTick, debugLabel: debugLabel);
 
-  final TickerProviderStateMixin<StatefulWidget> _creator;
+  final TickerProviderStateMixin _creator;
 
   @override
   void dispose() {