anchor.diff:    void _initSelection(TabBarSelectionState<T> newSelection) {
       if (_selection == newSelection)
         return;
  -    _selection?.unregisterAnimationListener(this);
  +    _selection?.removeAnimationListener(this);
       _selection = newSelection;
  -    _selection?.registerAnimationListener(this);
  +    _selection?.addAnimationListener(this);
       if (_selection != null)
         _lastSelectedIndex = _selection.index;
     }
  
positive.code:   void _initSelection(TabBarSelectionState<T> newSelection) {
      if (_selection == newSelection)
        return;
      _selection?.unregisterAnimationListener(this);
      _selection = newSelection;
      _selection?.registerAnimationListener(this);
      if (_selection != null)
        _updateItemsAndScrollBehavior();
    }
  
negative.code: 
    /// Stop calling listener methods every time the value or status of the animation changes.
    ///
    /// Listeners can be added with [addAnimationListener].
    void removeAnimationListener(TabBarSelectionAnimationListener listener) {
      _animationListeners.remove(listener);
      _controller
        ..removeStatusListener(listener.handleStatusChange)
  