anchor.diff:    String _title = '';
     Element _titleElement;
     FormElement _body;
  +  Element _progress;
     Iterable<SparkDialogButton> _buttons;
     List<_ValidatedField> _validatedFields = [];
   
  
positive.code: 
    void set activityVisible(bool visible) {
      _activityVisible = visible;
      $['progress'].classes.toggle('hidden', !_activityVisible);
    }
  
    void _addValidatableFields(Iterable<Node> candidates) {
  
negative.code:     _titleElement = $['title'];
      _body = $['body'];
      _buttons = SparkWidget.inlineNestedContentNodes($['buttonsContent']);
      _progress = $['progress'];
  
      title = _title;
      _progress.classes.toggle('hidden', !_activityVisible);
  
      // TODO(ussuri): Use MutationObserver here to detect added/removed fields.
      _addValidatableFields(
  