E single

Source

E get single {
  // Note that this throws correctly if the queue is empty
  // because reading the element of the sentinel throws.
  if (identical(_sentinel._nextLink, _sentinel._previousLink)) {
    _DoubleLinkedQueueEntry<E> entry = _sentinel._nextLink;
    return entry._element;
  }
  throw IterableElementError.tooMany();
}