Add @override annotation to known overriden methods

sky/sdk/example/fitness/lib/main.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/theme/colors.dart' as colors;
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/navigator.dart';
 import 'package:sky/widgets/theme.dart';
 import 'package:sky/widgets/widget.dart';
 
 import 'measurement.dart';
 import 'home.dart';
 import 'settings.dart';
 import 'fitness_types.dart';
 
 class FitnessApp extends App {
 
   NavigationState _navigationState;
   FitnessApp() {
     _navigationState = new NavigationState([
       new Route(
-        name: '/', 
+        name: '/',
         builder: (navigator, route) => new HomeFragment(navigator, _userData)
       ),
       new Route(
         name: '/settings',
         builder: (navigator, route) => new SettingsFragment(navigator, backupSetting, settingsUpdater)
       ),
     ]);
   }
 
+  @override
   void onBack() {
     if (_navigationState.hasPrevious()) {
       setState(() {
         _navigationState.pop();
       });
     } else {
       super.onBack();
     }
   }
 
   BackupMode backupSetting = BackupMode.disabled;
 
   void settingsUpdater({ BackupMode backup }) {
     setState(() {
       if (backup != null)
         backupSetting = backup;
     });
   }
 
   final List<Measurement> _userData = [
     new Measurement(when: new DateTime.now(), weight: 400.0)
   ];
 
+  @override
   Widget build() {
     return new Theme(
       data: new ThemeData(
         brightness: ThemeBrightness.light,
         primarySwatch: colors.Indigo,
         accentColor: colors.PinkAccent[200]
       ),
       child: new Navigator(_navigationState)
     );
   }
 }
 
 void main() {
   runApp(new FitnessApp());
 }