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:   final demo = new QrDemo(canvas, typeDiv, errorDiv);
  
    final InputElement input = query('#input');
  
    input.value = "Type your message in here...";
    demo.value = input.value;
  
  