testHighlightItemBehavior() {
    const node = dom.getElement('demoMenuButton');
    menuButton.decorate(node);
    testingEvents.fireClickSequence(node);

    assertTrue('Menu must open after click', menuButton.isOpen());

    menuButton.handleKeyEvent(new MyFakeEvent(KeyCodes.DOWN));
    assertNotNull(menuButton.getElement());
    assertEquals(
        'First menuitem must be the aria-activedescendant', 'menuItem1',
        aria.getState(menuButton.getElement(), State.ACTIVEDESCENDANT));

    menuButton.handleKeyEvent(new MyFakeEvent(KeyCodes.DOWN));
    assertEquals(
        'Second menuitem must be the aria-activedescendant', 'menuItem2',
        aria.getState(menuButton.getElement(), State.ACTIVEDESCENDANT));
  }
Open the menu, highlight first menuitem and then the second.
Check if the aria-activedescendant property is set correctly.
@suppress {checkTypes} suppression added to enable type checking