code: function queryRequiredElement(selectors, opt_context) {
    var element = (opt_context || document).querySelector(selectors);
    return assertInstanceof(element, HTMLElement,
                            'Missing required element: ' + selectors);
  }
docs: Query an element that's known to exist by a selector. We use this instead of
  just calling querySelector and not checking the result because this lets us
  satisfy the JSCompiler type system.
  @param {string} selectors CSS selectors to query the element.
  @param {(!Document|!DocumentFragment|!Element)=} opt_context An optional
  context object for querySelector.
  @return {!HTMLElement} the Element.