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