toPlainObject() {
    return {
      'email': this.email_,
      'displayName': this.displayName_,
      'photoUrl': this.photoUrl_,
      'providerId': this.providerId_
    };
  }
@return {{
  email: string,
  displayName: ?string,
  photoUrl: ?string,
  providerId: ?string
}} The plain object representation for the account.