anchor.diff:    }
   
     // abstract
  -  void drawOverride(CanvasRenderingContext2D ctx){
  -    throw "should override in subclass";
  -  }
  +  abstract void drawOverride(CanvasRenderingContext2D ctx);
   
     void invalidateDraw(){
       if(_lastDrawSize != null){
  
positive.code:   final List<core.AffineTransform> _transforms;
    final bool cacheEnabled;
    final core.EventHandle<core.EventArgs> _updatedEventHandle;
  
    num _width, _height, _alpha;
    core.Size _lastDrawSize;
  
negative.code: 
    PElement(this._width, this._height, [this.cacheEnabled = false]) :
      _transforms = new List<core.AffineTransform>(),
      _updatedEventHandle = new core.EventHandle<core.EventArgs>();
  
    num get width() => _width;
  
  