function requestViaLinkPrefetch(url) {
  var link = document.createElement('link');
  if (link.relList && link.relList.supports && link.relList.supports("prefetch")) {
    return createRequestViaElement("link",
                                   {"rel": "prefetch", "href": url},
                                   document.head);
  } else {
    return Promise.reject("This browser does not support 'prefetch'.");
  }
}
Creates a new link element for a prefetch, binds load and error events, sets
    the href to url and appends it to {@code document.head}.
@param {string} url The URL of a resource to prefetch.
@return {Promise} The promise for success/error events.