anchor.diff:  
   void main() {
     _process();
  +
  +  zoomBtn.onClick.listen((_) {
  +    if (_root != null) {
  +      _root.classes.toggle('zoom');
  +    }
  +  });
   }
   
   void _process() {
  +  if (_root != null) {
  +    _root.remove();
  +    _root = null;
  +  }
  +
     var removedLines = <String>[];
   
     var lines = _dotContent.where((line) {
  
positive.code:   zoomBtn.style.display = 'block';
  
    _root = querySelector('svg') as svg.SvgElement;
  
    zoomBtn.onClick.listen((_) {
      _root.classes.toggle('zoom');
    });
  
    for (var element in _root.querySelectorAll('g.node')) {
      var title = element.querySelector('title').text;
  
negative.code: }
  
  void _updateBody(String output) {
    assert(_root == null);
  
    output = LineSplitter
        .split(output)
  