anchor.diff:      _dragger.dragStart.add(_onDragStart);
   
       _demoMapper.outputChanged.add((e) => requestFrame());
  +
  +    _stage.invalidated.add(_onStageInvalidated);
     }
   
     void requestFrame(){
  
positive.code:       _frameRequested = true;
        window.requestAnimationFrame(_onFrame);
      }
    }
  
    void _onDrag(core.Vector delta) {
  
negative.code: 
    factory DraggerDemo(CanvasElement canvas){
  
      final image =
          new ImgElement.fromUrl('http://sweeper.j832.com/dart_logo.jpg',
              100, 100);
  
      var tx = image.addTransform();
  
      var rootPanel = new PCanvas(500, 500);
      rootPanel.addElement(image);
  
      var stage = new Stage(canvas, rootPanel);
      var dragger = new Dragger(canvas);
  