anchor.diff:   * A sentinel does not box any user element.
    */
   class _DoubleLinkedQueueSentinel<E> extends _DoubleLinkedQueueEntry<E> {
  -  _DoubleLinkedQueueSentinel(DoubleLinkedQueue<E> queue) : super(queue) {
  +  _DoubleLinkedQueueSentinel(DoubleLinkedQueue<E> queue)
  +      : super(null, queue) {
       _previousLink = this;
       _nextLink = this;
     }
  
positive.code: 
    E get first {
      _DoubleLinkedQueueEntry<E> firstEntry = _sentinel._nextLink;
      return firstEntry.element;
    }
  
    E get last {
      _DoubleLinkedQueueEntry<E> lastEntry = _sentinel._previousLink;
      return lastEntry.element;
    }
  
    E get single {
  
negative.code:   bool remove(Object o) {
      _DoubleLinkedQueueEntry<E> entry = _sentinel._nextLink;
      while (!identical(entry, _sentinel)) {
        if (entry._element == o) {
          entry._remove();
          _elementCount--;
          return true;
  