anchor.diff:  }
   
   void _updateBody(String output) {
  -  if (_root != null) {
  -    _root.remove();
  -  }
  +  assert(_root == null);
   
     output = LineSplitter
         .split(output)
  
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 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) {
  