anchor.diff:    final EventHandle<EventArgs> _invalidatedEventHandle = new EventHandle<EventArgs>();
     CanvasElement _cacheCanvas;
   
  -  num _width, _height, _alpha;
  +  num _width, _height, _alpha = 1;
     Size _lastDrawSize;
     bool clip = false;
     ElementParent _parent;
  
positive.code:   }
  
    void _drawInternal(CanvasRenderingContext2D ctx){
      if (_alpha != null) {
        ctx.globalAlpha = _alpha;
      }
  
  
negative.code:     assert(value.isValid);
      _width = value.width;
      _height = value.height;
      invalidateDraw();
    }
  
    num get alpha => _alpha;
  
    void set alpha(num value) {
      requireArgument(isValidNumber(value), 'value');
      requireArgument(value >= 0 && value <= 1, 'value');
      _alpha = value;
      invalidateDraw();
    }
  
  