anchor.diff:  part of dartrocket;
   
  -class Background {
  -  List _backgroundTileList = new List();
  +class Background implements StageXL.Animatable{
  +  List<StageXL.Bitmap> _backgroundTileList = new List<StageXL.Bitmap>();
     State _context;
  +  
  +  int vx = 0;
  +  int vy = 0;
  +  
  +  bool isMoveAble;
   
  -  Background(State stateContext, String resourceName, {bool addToStage:
  +  Background(State stateContext, String resourceName, {bool isMoveable:false,bool addToStage:
         true, bool repeatX: true, bool repeatY: true}) : super() {
  -    
  +        
       _context = stateContext;
       
       StageXL.BitmapData backgroundTileBitmapdata =
  
positive.code:       }
      }
      
      if (addToStage) {
        this.addToStage();
      }
  
negative.code:   addToStage() {
      _backgroundTileList.forEach((tile){
        _context.game.stage.addChild(tile);
      });
      if(isMoveAble){
        _context.game.stage.juggler.add(this);
      }
    }
  
    @override
    bool advanceTime(num time) {
      _backgroundTileList.forEach((tile){            
        tile.x = tile.x + vx * time;
        tile.y = tile.y + vy * time;
        
        if(tile.x >= _context.game.stage.sourceWidth){
          tile.x = tile.x - _context.game.stage.sourceWidth - tile.width;
        }else if(tile.y >= _context.game.stage.sourceHeight){
          tile.y = tile.y - _context.game.stage.sourceHeight - tile.height;        
        }      
      });
      
    }
  }
  