testCheckboxDisabled() {
    const node = dom.getElement('demoSelectionMenuButton');
    selectionMenuButton.decorate(node);

    const checkbox = dom.getElement('demoCheckbox');
    assertFalse('Checkbox must be enabled', checkbox.disabled);

    selectionMenuButton.setEnabled(false);
    assertTrue('Checkbox must be disabled', checkbox.disabled);

    selectionMenuButton.setEnabled(true);
    assertFalse('Checkbox must be enabled', checkbox.disabled);
  }
Tests that the checkbox gets disabled when the button is disabled
     @suppress {strictMissingProperties} suppression added to enable type
     checking