Add @override annotation to known overriden methods

sky/sdk/lib/widgets/navigator.dart

Index: sky/sdk/lib/widgets/navigator.dart
diff --git a/sky/sdk/lib/widgets/navigator.dart b/sky/sdk/lib/widgets/navigator.dart
index dbb150660ae3d19d90a4a21a361d3ec5fb8fba76..37dc9b53a98ab86848eed7245b538831def3e5be 100644
--- a/sky/sdk/lib/widgets/navigator.dart
+++ b/sky/sdk/lib/widgets/navigator.dart
@@ -16,6 +16,8 @@ abstract class RouteBase {
 class Route extends RouteBase {
   Route({ String name, this.builder }) : super(name: name);
   final Builder builder;
+
+  @override
   Widget build(Navigator navigator, RouteBase route) => builder(navigator, route);
 }
 
@@ -26,8 +28,10 @@ class RouteState extends RouteBase {
   RouteBase route;
   Function callback;
 
+  @override
   Widget build(Navigator navigator, _) => route.build(navigator, this);
 
+  @override
   void popState() {
     if (callback != null)
       callback(this);
@@ -80,6 +84,7 @@ class Navigator extends StatefulComponent {
 
   NavigationState state;
 
+  @override
   void syncFields(Navigator source) {
     state = source.state;
   }
@@ -113,6 +118,7 @@ class Navigator extends StatefulComponent {
     });
   }
 
+  @override
   Widget build() {
     return state.currentRoute.build(this, state.currentRoute);
   }