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