anchor.diff:    /// and they can be iterated using [DoubleLinkedQueueEntry.nextEntry()] and
     /// [DoubleLinkedQueueEntry.previousEntry()].
     DoubleLinkedQueueEntry<E> lastEntry() {
  -    return _sentinel.previousEntry();
  +    return _sentinel.previousEntry()!;
     }
   
     bool get isEmpty {
  
positive.code:   /// and they can be iterated using [DoubleLinkedQueueEntry.nextEntry()] and
    /// [DoubleLinkedQueueEntry.previousEntry()].
    DoubleLinkedQueueEntry<E> firstEntry() {
      return _sentinel.nextEntry();
    }
  
    /// The entry object of the last element in the queue.
  
negative.code: 
    E get _element => super._element as E;
  
    DoubleLinkedQueueEntry<E>? nextEntry() {
      _DoubleLinkedQueueEntry<E> entry = _nextLink as _DoubleLinkedQueueEntry<E>;
      return entry._asNonSentinelEntry();
    }
  
    DoubleLinkedQueueEntry<E>? previousEntry() {
      _DoubleLinkedQueueEntry<E> entry =
          _previousLink as _DoubleLinkedQueueEntry<E>;
      return entry._asNonSentinelEntry();
    }
  }
  
  