anchor.diff:  
   class _MapletSingleIterator<K> implements Iterator<K> {
     var _element;
  -  K _current;
  +  K? _current;
   
     _MapletSingleIterator(this._element);
   
     @override
  -  K get current => _current;
  +  K get current => _current as K;
   
     @override
     bool moveNext() {
  
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:   }
  
    @override
    V? remove(Object? key) {
      if (_extra == null) {
        if (_key != key) return null;
        _key = _MARKER;
  