it('should initialize a list from an iterable', () => {
   let list1 = LinkedList.from([0, 1, 2, 3]);
   let list2 = LinkedList.from(list1);
   expect(list2.isEmpty).to.equal(false);
   expect(list2.length).to.equal(4);
   expect(list2.first).to.equal(0);
   expect(list2.last).to.equal(3);
   expect(toArray(list2)).to.deep.equal([0, 1, 2, 3]);
 });

  /**
   * Run an iteration of the message loop.
   *
   * This will process all pending messages in the queue. If a message
   * is added to the queue while the message loop is running, it will
   * be processed on the next cycle of the loop.
   */
  function runMessageLoop(): void {
    // Clear the task ID so the next loop can be scheduled.
    loopTaskID = 0;

    // If the message queue is empty, there is nothing else to do.
    if (messageQueue.isEmpty) {
      return;
    }

    // Add a sentinel value to the end of the queue. The queue will
    // only be processed up to the sentinel. Messages posted during
    // this cycle will execute on the next cycle.
    let sentinel: PostedMessage = { handler: null, msg: null };
    messageQueue.addLast(sentinel);

    // Enter the message loop.
    while (true) {
      // Remove the first posted message in the queue.
      let posted = messageQueue.removeFirst()!;

      // If the value is the sentinel, exit the loop.
      if (posted === sentinel) {
        return;
      }

      // Dispatch the message if it has not been cleared.
      if (posted.handler && posted.msg) {
        sendMessage(posted.handler, posted.msg);
      }
    }
  }

  /**
   * Add a message to the end of the message queue.
   *
   * This will automatically schedule a run of the message loop.
   */
  function enqueueMessage(handler: IMessageHandler, msg: Message): void {
    // Add the posted message to the queue.
    messageQueue.addLast({ handler, msg });

    // Bail if a loop task is already pending.
    if (loopTaskID !== 0) {
      return;
    }

    // Schedule a run of the message loop.
    loopTaskID = schedule(runMessageLoop);
  }