async loadText_(url) {
    const netEngine = new shaka.net.NetworkingEngine();
    const retryParams = shaka.net.NetworkingEngine.defaultRetryParameters();
    const request = shaka.net.NetworkingEngine.makeRequest([url], retryParams);
    const requestType = shaka.net.NetworkingEngine.RequestType.APP;
    const operation = netEngine.request(requestType, request);
    const response = await operation.promise;
    const text = shaka.util.StringUtils.fromUTF8(response.data);
    await netEngine.destroy();
    return text;
  }
@param {string} url
@return {!Promise<string>}
@private