anchor.diff:    }
   
     void _reset() {
  +    _appendAfter = null;
  +
       if (this.isDirty) {
         // Map state before changes.
         _previousMapHead = _mapHead;
  
positive.code:   KeyValueChangeRecord _additionsTail;
  
    KeyValueChangeRecord _removalsHead;
  
    // ignore: unused_field
    KeyValueChangeRecord _removalsTail;
  
    bool get isDirty {
      return !identical(this._additionsHead, null) ||
  
negative.code:     KeyValueChangeRecord record;
      for (record = this._removalsHead;
          !identical(record, null);
          record = record._next) {
        fn(record);
      }
    }
  