testAlternatePositioningElement() {
    const node = dom.getElement('demoMenuButton');
    menuButton.decorate(node);

    const posElement = dom.getElement('positionElement');
    menuButton.setPositionElement(posElement);

    // Show the menu.
    menuButton.setOpen(true);

    // Confirm the menu position
    const menuNode = menuButton.getMenu().getElement();
    assertRoughlyEquals(
        menuNode.offsetTop, posElement.offsetTop + posElement.offsetHeight, 20);
    assertRoughlyEquals(menuNode.offsetLeft, posElement.offsetLeft, 20);
  }
Use a different button to position the menu and make sure it does so
correctly.
@suppress {strictMissingProperties} suppression added to enable type
checking