Fix bugs found by Dart analyzer

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

 part of sprites;
 
 /// Options for setting up a [SpriteBox].
 ///
 ///  * [nativePoints], use the same points as the parent [Widget].
 ///  * [letterbox], use the size of the root node for the coordinate system, constrain the aspect ratio and trim off
 ///  areas that end up outside the screen.
 ///  * [stretch], use the size of the root node for the coordinate system, scale it to fit the size of the box.
 ///  * [scaleToFit], similar to the letterbox option, but instead of trimming areas the sprite system will be scaled
 ///  down to fit the box.
@@ skipping 248 matching lines @@
     canvas.concat(transformMatrix.storage);
 
     // Draw the sprite tree
     _rootNode._visit(canvas);
 
     canvas.restore();
   }
 
   // Updates
 
-  int _animationId = 0;
-
   void _scheduleTick() {
-    _animationId = scheduler.requestAnimationFrame(_tick);
+    scheduler.requestAnimationFrame(_tick);
   }
 
   void _tick(double timeStamp) {
 
       // Calculate the time between frames in seconds
     if (_lastTimeStamp == null) _lastTimeStamp = timeStamp;
     double delta = (timeStamp - _lastTimeStamp) / 1000;
     _lastTimeStamp = timeStamp;
 
     // Count the number of frames we've been running
@@ skipping 102 matching lines @@
   ///     if (event.pointer == firstPointerId) {
   ///       // Do something
   ///     }
   final int pointer;
 
   /// Creates a new SpriteBoxEvent, typically this is done internally inside the SpriteBox.
   ///
   ///     var event = new SpriteBoxEvent(new Point(50.0, 50.0), 'pointerdown', 0);
   SpriteBoxEvent(this.boxPosition, this.type, this.pointer);
 }