anchor.diff:    final List _list;
     int _remaining;
     int _index = 0;
  -  K _current;
  +  K? _current;
   
     _MapletListIterator(this._list, this._remaining);
   
     @override
  -  K get current => _current;
  +  K get current => _current as K;
   
     @override
     bool moveNext() {
  
positive.code: // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
  // for details. All rights reserved. Use of this source code is governed by a
  // BSD-style license that can be found in the LICENSE file.
  
  library dart2js.util.maplet;
  
  
negative.code:     } else {
        int remaining = _extra;
        int index = 0;
        int copyTo = 0;
        int copyFrom = 0;
        while (remaining > 0 && index < CAPACITY) {
          var candidate = _key[index];
          if (_MARKER == candidate) {
  