anchor.diff:  
       _canvas = canvas,
       _ctx = canvas.context2d,
  -    _qrMapper = new _QrCalc() {
  +    _qrMapper = new _QrCalc(),
  +    _scale = new core.BungeeNum(1) {
       _ctx.fillStyle = 'black';
   
       _qrMapper.outputChanged.add((args) {
  
positive.code:     final scale = minDimension ~/ size;
      final offset = (minDimension - (scale * size)) ~/ 2;
  
      _ctx.setTransform(scale, 0, 0, scale, offset, offset);
  
      if(_squares.length > 0) {
        assert(_squares.length == size * size);
  
negative.code: class QrDemo{
    static final String _typeRadioIdKey = 'type-value';
    static final String _errorLevelIdKey = 'error-value';
    final core.BungeeNum _scale;
    final CanvasElement _canvas;
    final _QrCalc _qrMapper;
    final CanvasRenderingContext2D _ctx;
  