function requestViaLinkStylesheet(url) {
  return createRequestViaElement("link",
                                 {"rel": "stylesheet", "href": url},
                                 document.head);
}
Creates a new link element for a stylesheet, binds load and error events,
    sets the href to url and appends it to {@code document.head}.
@param {string} url The URL for a stylesheet.
@return {Promise} The promise for success/error events.