First pass on action animations for sprites

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

 part of game;
 
 const double _steeringThreshold = 0.0;
 const double _steeringMax = 150.0;
 
 // Random generator
 Math.Random _rand = new Math.Random();
 
 const double _gameSizeWidth = 1024.0;
 const double _gameSizeHeight = 1024.0;
@@ skipping 285 matching lines @@
 
   Asteroid(Texture img, AsteroidSize this._asteroidSize) : super(img) {
     size = new Size(radius * 2.0, radius * 2.0);
     position = new Point(_gameSizeWidth * _rand.nextDouble(), _gameSizeHeight * _rand.nextDouble());
     rotation = 360.0 * _rand.nextDouble();
 
     _movementVector = new Point(_rand.nextDouble() * _maxAsteroidSpeed * 2 - _maxAsteroidSpeed,
                                 _rand.nextDouble() * _maxAsteroidSpeed * 2 - _maxAsteroidSpeed);
 
     userInteractionEnabled = true;
+
+    // Rotate forever
+    double direction = (_rand.nextBool()) ? 360.0 : -360.0;
+    ActionTween rot = new ActionTween( (a) => rotation = a, 0.0, direction, 2.0 * _rand.nextDouble() + 2.0);
+    ActionRepeatForever repeat = new ActionRepeatForever(rot);
+    actions.run(repeat);
   }
 
   bool handleEvent(SpriteBoxEvent event) {
     if (event.type == "pointerdown") {
       colorOverlay = new Color(0x99ff0000);
     }
     else if (event.type == "pointerup") {
       colorOverlay = null;
     }
     return false;
@@ skipping 151 matching lines @@
   double dy = a.y - b.y;
   if (dx < 0.0) dx = -dx;
   if (dy < 0.0) dy = -dy;
   if (dx > dy) {
     return dx + dy/2.0;
   }
   else {
     return dy + dx/2.0;
   }
 }