testOpen_notUnsent() {
    mockControl.$replayAll();

    const xhr = factory.createInstance();
    xhr.open('GET', 'https://www.google.com', true /* opt_async */);
    assertThrows(() => {
      xhr.open('GET', 'https://www.google.com', true /* opt_async */);
    });

    mockControl.$verifyAll();
  }
Verifies the open method when the ready state is not unsent.