getAuthDomain() {
    const authDomain = this.config_.get('authDomain');
    if (!authDomain) {
      throw new Error('Invalid project configuration: authDomain is required!');
    }
    return /** @type {string} */ (authDomain);
  }
Returns the project-level Auth domain. Throws error if not provided
in the configuration.
@return {string} The Auth domain.