anchor.diff:  
   // Background starfield
   
  -class StarField extends Node {
  -  Image _image;
  +class StarField extends NodeWithSize {
  +  sky.Image _image;
     int _numStars;
  +  bool _autoScroll;
     List<Point> _starPositions;
     List<double> _starScales;
     List<Rect> _rects;
  
positive.code:       if (yPos < 0) yPos += _gameSizeHeight;
  
        _starPositions[i] = new Point(xPos, yPos);
      }
    }
  }
  
negative.code:     ..isAntiAlias = false
      ..setTransferMode(sky.TransferMode.plus);
  
    StarField(SpriteSheet spriteSheet, this._numStars, [this._autoScroll = false]) : super(new Size(1024.0, 1024.0)) {
      _starPositions = [];
      _starScales = [];
      _colors = [];
  