anchor.diff:  main(){
     CanvasElement canvas = document.query("#content");
     final demo = new QrDemo(canvas);
  +  demo._setValue("hello world");
     demo.requestFrame();
   }
   
   class QrDemo{
  +  static final int typeNumber = 10;
  +  static final int size = typeNumber * 4 + 17;
  +  static final int scale = 5;
  +
  +  final List<bool> _squares;
  +
     final CanvasElement _canvas;
  +  CanvasRenderingContext2D _ctx;
   
     core.Coordinate _mouseLocation;
     bool _frameRequested = false;
  
positive.code:     return new QrDemo._internal(canvas);
    }
  
    QrDemo._internal(this._canvas) {
      _canvas.on.mouseMove.add(_canvas_mouseMove);
      _canvas.on.mouseOut.add(_canvas_mouseOut);
    }
  
negative.code: 
    void _setMouse(core.Coordinate value) {
      _mouseLocation = value;
      //requestFrame();
    }
  }
  