anchor.diff:  // 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.
  +
  +// @dart=2.12
   
   library dart2js.util.maplet;
   
  
positive.code:   final List _list;
    int _remaining;
    int _index = 0;
    K _current;
  
    _MapletListIterator(this._list, this._remaining);
  
    @override
    K get current => _current;
  
    @override
    bool moveNext() {
  
negative.code: 
  class _MapletSingleIterator<K> implements Iterator<K> {
    var _element;
    K? _current;
  
    _MapletSingleIterator(this._element);
  
    @override
    K get current => _current as K;
  
    @override
    bool moveNext() {
  