anchor.diff:  
     Element _target;
     var _targetScrollSubscription;
  -  int _visibleCount;
     int _physicalCount;
  -  int _physicalHeight;
     int _scrollTop = 0;
     bool _oldMulti = false;
     bool _oldSelectionEnabled = false;
  
positive.code: 
    Expando _selectedData;
    Expando<_PhysicalItemData> _physicalItemData = new Expando();
  
    int firstPhysicalIndex;
    int baseVirtualIndex;
  
    Element template;
  
  
negative.code:    * @default core-list-dart
     */
    @published Element scrollTarget;
  
    /**
     * 
  