testStopEscapePropagationMenuOpen() {
    const node = dom.getElement('demoMenuButton');
    /** @suppress {checkTypes} suppression added to enable type checking */
    const fakeEvent = new MyFakeEvent(KeyCodes.ESCAPE, EventType.KEYDOWN);
    menuButton.decorate(node);
    menuButton.setOpen(true);

    menuButton.handleKeyDownEvent_(fakeEvent);
    assertTrue(
        'Event propagation was not stopped.', fakeEvent.propagationStopped);
  }
Tests that a keydown event of the escape key is prevented from propagating
when the menu is open.
@suppress {visibility} suppression added to enable type checking