anchor.diff:    ItemRecord<V> _remove(ItemRecord record) => _addToRemovals(_unlink(record));
   
     ItemRecord<V> _unlink(ItemRecord record) {
  -    _movedItems.remove(record);
  +    if (_movedItems != null) _movedItems.remove(record);
   
       var prev = record._prev;
       var next = record._next;
  
positive.code:       record.currentIndex = record.previousIndex = i;
        record._prev = prev;
        if (prev != null) prev._next = prev._nextPrevious = record;
        _movedItems.put(record);
      }
  
  
negative.code:   int _length;
  
    /// Keeps track of moved items.
    DuplicateMap _movedItems;
  
    /// Keeps track of removed items.
    DuplicateMap _removedItems;
  
    ItemRecord<V> _previousItHead;
    ItemRecord<V> _itHead, _itTail;
  