code: function storeOfflineLogin(credentials, serverOptions, loginResponse) {
      return cipher.encryptJson(credentials['password'], loginResponse).then(function (value) {
          localStorage().setItem(computeLocalStorageKey(serverOptions), JSON.stringify(value));
          return loginResponse;
      });
  }
docs: writes response data to persistent storage for offline login purposes.
  
  @param credentials required for encryption.
  @param serverOptions identifying the server.
  @param loginResponse permitted to durable storage.
  @return {Promise<http.LoginResponse>} indicating success or failure.
  
  @internal Not part of public API, for library use only.