Add @override annotation to known overriden methods

sky/sdk/example/game/lib/game_demo_world.dart

Index: sky/sdk/example/game/lib/game_demo_world.dart
diff --git a/sky/sdk/example/game/lib/game_demo_world.dart b/sky/sdk/example/game/lib/game_demo_world.dart
index 2110689fcba68082c569bcf00a13bf9ced381819..0ac4a40dffded24aca0b0bd1836fefcca40af0f4 100644
--- a/sky/sdk/example/game/lib/game_demo_world.dart
+++ b/sky/sdk/example/game/lib/game_demo_world.dart
@@ -47,7 +47,7 @@ class GameDemoWorld extends NodeWithSize {
   // Game state
   int _numFrames = 0;
   bool _isGameOver = false;
-  
+
   GameDemoWorld(this._app, ImageMap images, this._spriteSheet) : super(new Size(_gameSizeWidth, _gameSizeHeight)) {
 
     // Fetch images
@@ -86,7 +86,7 @@ class GameDemoWorld extends NodeWithSize {
   }
 
   // Methods for adding game objects
-  
+
   void addAsteroid(AsteroidSize size, [Point pos]) {
     Asteroid asteroid = new Asteroid(_spriteSheet, size);
     asteroid.zPosition = 1.0;
@@ -204,6 +204,7 @@ class GameDemoWorld extends NodeWithSize {
     _gameLayer.addChild(explosionNode);
   }
 
+  @override
   void update(double dt) {
     // Move asteroids
     for (Asteroid asteroid in _asteroids) {
@@ -347,6 +348,7 @@ class GameDemoWorld extends NodeWithSize {
   int _secondPointer = -1;
   Point _firstPointerDownPos;
 
+  @override
   bool handleEvent(SpriteBoxEvent event) {
 
     Point pointerPos = convertPointToNodeSpace(event.boxPosition);
@@ -454,6 +456,7 @@ class Asteroid extends Sprite {
     actions.run(repeat);
   }
 
+  @override
   bool handleEvent(SpriteBoxEvent event) {
     if (event.type == "pointerdown") {
       actions.stopWithTag("fade");