getProviderAdditionalScopes(providerId) {
    // Get provided sign-in options for specified provider.
    const signInOptions = this.getSignInOptionsForProvider_(providerId);
    const scopes = signInOptions && signInOptions['scopes'];
    return Array.isArray(scopes) ? scopes : [];
  }
@param {string} providerId The provider id whose additional scopes are to
    be returned.
@return {!Array<string>} The list of additional scopes for specified
    provider.