Add @override annotation to known overriden methods

sky/sdk/lib/widgets/theme.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 '../theme/theme_data.dart';
 import 'basic.dart';
 import 'widget.dart';
 
 export '../theme/theme_data.dart' show ThemeData, ThemeBrightness;
 
@@ skipping 10 matching lines @@
 
   final ThemeData data;
 
   static final ThemeData _kFallbackTheme = new ThemeData.fallback();
 
   static ThemeData of(Component component) {
     Theme theme = component.inheritedOfType(Theme);
     return theme == null ? _kFallbackTheme : theme.data;
   }
 
+  @override
   bool syncShouldNotify(Theme old) => data != old.data;
 
 }