test_setPopupParentElement() {
    const popupParentElement = dom.getElement('popupParent');
    setPopupParentElement(popupParentElement);
    assertEquals(
        'PopupDatePicker should be parented to the popupParent DIV',
        popupParentElement, popupDatePicker.getElement().parentNode);
  }
Ensure that if setPopupParentElement is called, that the
PopupDatePicker is parented to the specified element.