testSetOpenUnsetsAriaHidden() {
    const node = dom.getElement('demoMenuButton');
    menuButton.decorate(node);
    const menuElem = menuButton.getMenu().getElementStrict();
    aria.setState(menuElem, State.HIDDEN, true);
    menuButton.setOpen(true);
    assertEquals('', aria.getState(menuElem, State.HIDDEN));
  }
Tests that the attached menu has been set to aria-hidden=false explicitly
when the menu is opened.