Add @override annotation to known overriden methods

sky/sdk/lib/widgets/material.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import '../animation/animation_performance.dart';
 import '../painting/box_painter.dart';
 import 'animated_component.dart';
 import 'animated_container.dart';
 import 'basic.dart';
 import 'default_text_style.dart';
@@ skipping 29 matching lines @@
     watchPerformance(_container.createPerformance(
         _container.shadow, duration: _kAnimateShadowDuration));
     watchPerformance(_container.createPerformance(
         _container.backgroundColor, duration: _kAnimateColorDuration));
   }
 
   Widget child;
 
   AnimatedContainer _container;
 
+  @override
   void syncFields(Material source) {
     child = source.child;
     _container.syncFields(source._container);
     super.syncFields(source);
   }
 
   Color _getBackgroundColor(MaterialType type, Color color) {
     if (color != null)
       return color;
     switch (type) {
       case MaterialType.canvas:
         return Theme.of(this).canvasColor;
       case MaterialType.card:
         return Theme.of(this).cardColor;
       default:
         return null;
     }
   }
 
   Widget build() {
     return _container.build(
         new DefaultTextStyle(style: Theme.of(this).text.body1, child: child)
     );
   }
 
 }