Add @override annotation to known overriden methods

sky/sdk/example/fitness/lib/settings.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 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/switch.dart';
 import 'package:sky/widgets/icon_button.dart';
 import 'package:sky/widgets/material.dart';
 import 'package:sky/widgets/menu_item.dart';
 import 'package:sky/widgets/navigator.dart';
@@ skipping 50 matching lines @@
                 new Text('Height'),
                 new Text("6'2\"", style: Theme.of(this).text.caption),
               ])
             ]
           ),
         ])
       )
     );
   }
 
+  @override
   Widget build() {
     return new Scaffold(
       toolbar: buildToolBar(),
       body: buildSettingsPane()
     );
   }
 }