getProviderCustomParameters(providerId) {
    // Get provided sign-in options for specified provider.
    const signInOptions = this.getSignInOptionsForProvider_(providerId);
    // Get customParameters for that provider if available.
    const customParameters = signInOptions && signInOptions['customParameters'];
    // Custom parameters must be an object.
    if (goog.isObject(customParameters)) {
      // Clone original custom parameters.
      const clonedCustomParameters = googObject.clone(customParameters);
      // Delete login_hint from Google provider as it could break the flow.
      if (providerId === firebase.auth.GoogleAuthProvider.PROVIDER_ID) {
        delete clonedCustomParameters['login_hint'];
      }
      // Delete login from GitHub provider as it could break the flow.
      if (providerId === firebase.auth.GithubAuthProvider.PROVIDER_ID) {
        delete clonedCustomParameters['login'];
      }
      return clonedCustomParameters;
    }
    return null;
  }
@param {string} providerId The provider id whose custom parameters are to
    be returned.
@return {?Object} The custom parameters for the current provider.