http://meowni.ca/posts/web-fonts/ https://github.com/GoogleChrome/devsummit/blob/master/scripts/utils.js#L34

async_lazy_load_assets.js

/*!
 *
 * Copyright 2016 Google Inc. All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

'use strict';

export function loadScript (url, async) {
  return new Promise(function (resolve, reject) {
    var script = document.createElement('script');
    script.src = url;
    if (typeof async !== 'undefined') {
      script.async = async;
    }

    script.onerror = reject;
    script.onload = resolve;
    document.head.appendChild(script);
  });
}

export function loadStyles (url) {
  return new Promise(function (resolve, reject) {
    var xhr = new XMLHttpRequest();
    xhr.returnType = 'text';
    xhr.onload = function () {
      var link = document.createElement('link');
      link.rel = 'stylesheet';
      link.href = url;
      document.head.appendChild(link);
      resolve();
    };
    xhr.onerror = reject;
    xhr.open('get', url);
    xhr.send();
  });
}