anchor.diff:     */
     void forEach(void action(E entry)) {
       int modificationCount = _modificationCount;
  -    _LinkedListLink current = _next;
  +    E current = _next;
       while (!identical(current, this)) {
         action(current);
         if (modificationCount != _modificationCount) {
  
positive.code:     _modificationCount++;
      _LinkedListLink next = _next;
      while (!identical(next, this)) {
        _LinkedListLink entry = next;
        next = entry._next;
        entry._next = entry._previous = entry._list = null;
      }
  
negative.code:    * Return the succeeding element in the list.
     */
    E get next {
      if (identical(_next, _list)) return null;
      return _next as E;
    }
  
    /**
     * Return the preceeding element in the list.
     */
    E get previous {
      if (identical(_previous, _list)) return null;
      return _previous as E;
    }
  
    /**
  