constructor(email, credential) {
    /** @const @private {string} The pending email. */
    this.email_ = email;
    /**
     * @const @private {?firebase.auth.AuthCredential} The pending credential.
     */
    this.credential_ = credential || null;
  }
@param {string} email The pending email.
@param {?firebase.auth.AuthCredential=} credential The pending auth
    credential.