public T pop() {
    // NULLSAFE_FIXME[Nullable Dereference]
    if (head.delay > 0) {
      throw new IllegalStateException("cannot pop the head element when it has a non-zero delay");
    }

    // NULLSAFE_FIXME[Nullable Dereference]
    T popped = head.value;
    // NULLSAFE_FIXME[Nullable Dereference]
    head = head.next;
    size--;
    return popped;
  }
Pops the next element off the queue. Only valid to call if the head is ready to be pop'd
because of the passage of time.

@return the next element off the queue.
imagepipeline-base-test/src/main/java/com/facebook/imagepipeline/testing/DeltaQueue.java