load(url, callback, errback) {
    var xhr = new XMLHttpRequest();
    xhr.onload = () => {
      if (xhr.status == 200 || xhr.status == 0) {
        callback(xhr.responseText);
      } else {
        var err;
        if (xhr.status === 404)
          err = 'File not found \'' + url + '\'';
        else
          err = xhr.status + xhr.statusText;
        errback(err);
      }
      xhr = null;
    };
    xhr.onerror = (err) => {
      errback(err);
    };
    xhr.open('GET', url, true);
    xhr.send();
    return () => {
      xhr && xhr.abort();
    };
  }
@return {Function} A function that aborts the async loading.