testOpen() {
    mockControl.$replayAll();

    const xhr = factory.createInstance();
    assertEquals(0, xhr.status);
    assertEquals('', xhr.responseText);
    assertEquals(xhr.readyState, FetchXmlHttp.RequestState.UNSENT);

    /** @suppress {checkTypes} suppression added to enable type checking */
    const onReadyStateChangeHandler = new recordFunction();
    xhr.onreadystatechange = onReadyStateChangeHandler;
    xhr.open('GET', 'https://www.google.com', true /* opt_async */);
    assertEquals(xhr.readyState, FetchXmlHttp.RequestState.OPENED);
    onReadyStateChangeHandler.assertCallCount(1);

    mockControl.$verifyAll();
  }
Verifies the open method. @suppress {checkTypes} suppression added to
     enable type checking