Add @override annotation to known overriden methods

sky/sdk/example/fitness/lib/main.dart

Index: sky/sdk/example/fitness/lib/main.dart
diff --git a/sky/sdk/example/fitness/lib/main.dart b/sky/sdk/example/fitness/lib/main.dart
index 6d447a831e4a8599e832fd43f7d43f8ab0e5e770..9e6d0e706b357c249d6022aa5ea1410bd4464029 100644
--- a/sky/sdk/example/fitness/lib/main.dart
+++ b/sky/sdk/example/fitness/lib/main.dart
@@ -19,7 +19,7 @@ class FitnessApp extends App {
   FitnessApp() {
     _navigationState = new NavigationState([
       new Route(
-        name: '/', 
+        name: '/',
         builder: (navigator, route) => new HomeFragment(navigator, _userData)
       ),
       new Route(
@@ -29,6 +29,7 @@ class FitnessApp extends App {
     ]);
   }
 
+  @override
   void onBack() {
     if (_navigationState.hasPrevious()) {
       setState(() {
@@ -52,6 +53,7 @@ class FitnessApp extends App {
     new Measurement(when: new DateTime.now(), weight: 400.0)
   ];
 
+  @override
   Widget build() {
     return new Theme(
       data: new ThemeData(