Add @override annotation to known overriden methods

sky/sdk/lib/widgets/animated_component.dart

Index: sky/sdk/lib/widgets/animated_component.dart
diff --git a/sky/sdk/lib/widgets/animated_component.dart b/sky/sdk/lib/widgets/animated_component.dart
index 839e336369628da0fdff1f04251300f3d17b0dc1..58ee10e99b66f7c8b1a7faec34a7a53aed6c7bbd 100644
--- a/sky/sdk/lib/widgets/animated_component.dart
+++ b/sky/sdk/lib/widgets/animated_component.dart
@@ -18,6 +18,7 @@ abstract class AnimatedComponent extends StatefulComponent {
 
   AnimatedComponent({ String key }) : super(key: key);
 
+  @override
   void syncFields(AnimatedComponent source) { }
 
   final List<_AnimationEntry> _animatedFields = new List<_AnimationEntry>();
@@ -39,6 +40,7 @@ abstract class AnimatedComponent extends StatefulComponent {
     _watchedPerformances.add(performance);
   }
 
+  @override
   void didMount() {
     for (_AnimationEntry entry in _animatedFields) {
       entry.subscription = entry.value.onValueChanged.listen((_) {
@@ -52,6 +54,7 @@ abstract class AnimatedComponent extends StatefulComponent {
     super.didMount();
   }
 
+  @override
   void didUnmount() {
     for (_AnimationEntry entry in _animatedFields) {
       assert(entry.subscription != null);