constructor(email, displayName, photoUrl, providerId) {
    this.email_ = email;
    this.displayName_ = displayName || null;
    this.photoUrl_ = photoUrl || null;
    this.providerId_ = providerId || null;
  }
@param {string} email The email address.
@param {?string=} displayName The display name, defaulting to null.
@param {?string=} photoUrl The profile photo URL, defaulting to null.
@param {?string=} providerId The identity provider ID, defaulting to
    null.