getSignInOptionsForProvider_(providerId) {
    const signInOptions = this.getSignInOptions_();
    // For each sign-in option.
    for (let i = 0; i < signInOptions.length; i++) {
      // Check if current option matches provider ID.
      if (signInOptions[i]['provider'] === providerId) {
        return signInOptions[i];
      }
    }
    return null;
  }
Returns the normalized signInOptions for the specified provider.
@param {string} providerId The provider id whose signInOptions are to be
    returned.
@return {?Object} The normalized sign-in options for the specified
    provider.
@private