anchor.diff:    }
   
     @override
  -  E lookup(Object element) {
  +  E? lookup(Object? element) {
       if (_extra == null) {
         return _contents == element ? _contents : null;
       } else if (_MARKER == _extra) {
  
positive.code:         // Do nothing.
          return false;
        } else {
          List list = new List.filled(CAPACITY, null);
          list[0] = _contents;
          list[1] = element;
          _contents = list;
  
negative.code:     } else {
        int remaining = _extra;
        int index = 0;
        int copyTo = 0;
        int copyFrom = 0;
        while (remaining > 0 && index < CAPACITY) {
          var candidate = _contents[index++];
          if (_MARKER == candidate) {
  