testActiveElement() {
    assertNull(mh.activeElement_);

    mockElement.dispatchEvent('keydown');
    assertEquals(mockElement, mh.activeElement_);

    mockElement.dispatchEvent('blur');
    assertNull(mh.activeElement_);

    mockElement.dispatchEvent('focus');
    assertEquals(mockElement, mh.activeElement_);

    mh.detachInput(mockElement);
    assertNull(mh.activeElement_);
  }
Make sure that the active element handling works.
@suppress {visibility} suppression added to enable type checking