anchor.diff:        _indicatorRect = new Rect.fromLTRB(
           leftRect.left, leftRect.top, rightRect.right, rightRect.bottom
         );
  -    } else {
  -      // TODO(abarth): If we've never gone through handleStatusChange before, we
  -      // might not have set up our _indicatorTween yet.
  -      _indicatorRect = _indicatorTween.lerp(t);
       }
       if (oldRect != _indicatorRect)
         setState(() { /* The indicator rect has changed. */ });
  
positive.code: 
      if (_valueIsChanging && status == AnimationStatus.completed) {
        _valueIsChanging = false;
        _indicatorTween
          ..begin = _tabIndicatorRect(math.max(0, _selection.index - 1))
          ..end = _tabIndicatorRect(math.min(config.labels.length - 1, _selection.index + 1));
        setState(() {
          _indicatorRect = _tabIndicatorRect(_selection.index);
        });
      }
  
negative.code:   Size _tabBarSize;
    List<double> _tabWidths;
    Rect _indicatorRect;
    RectTween _indicatorTween;
  
    Rect _tabRect(int tabIndex) {
      assert(_tabBarSize != null);
  