Add @override annotation to known overriden methods

sky/sdk/lib/animation/generators.dart

Index: sky/sdk/lib/animation/generators.dart
diff --git a/sky/sdk/lib/animation/generators.dart b/sky/sdk/lib/animation/generators.dart
index f7bf0f04ff16e8ff379a89db8df2e02073a408dd..2c2ebbf9804c9537301103b68519f9f247efb73e 100644
--- a/sky/sdk/lib/animation/generators.dart
+++ b/sky/sdk/lib/animation/generators.dart
@@ -17,6 +17,7 @@ class FrameGenerator extends Generator {
   Function onDone;
   StreamController _controller;
 
+  @override
   Stream<double> get onTick => _controller.stream;
 
   int _animationId = 0;
@@ -29,6 +30,7 @@ class FrameGenerator extends Generator {
       onCancel: cancel);
   }
 
+  @override
   void cancel() {
     if (_cancelled) {
       return;
@@ -58,6 +60,7 @@ class FrameGenerator extends Generator {
 }
 
 class AnimationGenerator extends Generator {
+  @override
   Stream<double> get onTick => _stream;
   final double initialDelay;
   final double duration;
@@ -102,6 +105,7 @@ class AnimationGenerator extends Generator {
     return duration - _lastTime;
   }
 
+  @override
   void cancel() {
     _generator.cancel();
   }
@@ -123,6 +127,7 @@ class AnimationGenerator extends Generator {
 }
 
 class Simulation extends Generator {
+  @override
   Stream<double> get onTick => _stream;
   final System system;
 
@@ -145,6 +150,7 @@ class Simulation extends Generator {
     }
   }
 
+  @override
   void cancel() {
     _generator.cancel();
   }