anchor.diff:  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;
  
positive.code: 
      _canvas = canvas,
      _ctx = canvas.context2d,
      _qrMapper = new _QrCalc() {
      _ctx.fillStyle = 'black';
  
      _qrMapper.outputChanged.add((args) {
  
negative.code:     final scale = minDimension ~/ size;
      final offset = (minDimension - (scale * size)) ~/ 2;
  
      _scale.target = scale;
  
      if(_scale.update()) {
        requestFrame();
      }
  
      _ctx.setTransform(_scale.current, 0, 0, _scale.current, offset, offset);
  
      if(_squares.length > 0) {
        assert(_squares.length == size * size);
  