anchor.diff:    }
   
     bool get isEmpty {
  -    return (identical(_sentinel._next, _sentinel));
  +    return (identical(_sentinel._nextLink, _sentinel));
     }
   
     void clear() {
  -    _sentinel._next = _sentinel;
  -    _sentinel._previous = _sentinel;
  +    _sentinel._nextLink = _sentinel;
  +    _sentinel._previousLink = _sentinel;
       _elementCount = 0;
     }
   
     void forEachEntry(void f(DoubleLinkedQueueEntry<E> element)) {
  -    _DoubleLinkedQueueEntry<E> entry = _sentinel._next;
  +    _DoubleLinkedQueueEntry<E> entry = _sentinel._nextLink;
       while (!identical(entry, _sentinel)) {
  -      _DoubleLinkedQueueEntry<E> nextEntry = entry._next;
  +      _DoubleLinkedQueueEntry<E> nextEntry = entry._nextLink;
         _DoubleLinkedQueueElement element = entry;
         f(element);
         entry = nextEntry;
  
positive.code:   E get element;
  
    DoubleLinkedQueueEntry<E> nextEntry() {
      _DoubleLinkedQueueEntry next = _next;
      return next._asNonSentinelEntry();
    }
  
    DoubleLinkedQueueEntry<E> previousEntry() {
      _DoubleLinkedQueueEntry previous = _previous;
      return previous._asNonSentinelEntry();
    }
  }
  
negative.code:  */
  class _DoubleLinkedQueueSentinel<E> extends _DoubleLinkedQueueEntry<E> {
    _DoubleLinkedQueueSentinel(DoubleLinkedQueue queue) : super(queue) {
      _previousLink = this;
      _nextLink = this;
    }
  
    _DoubleLinkedQueueElement _asNonSentinelEntry() {
  