code: function() {
          // We sign in the user through the normal federated sign-in flow,
          // and the callback handler will take care of linking the
          // pending credential to the successfully signed in user.
          // Pass the email since some OAuth providers support OAuth flow
          // with a specified email.
          firebaseui.auth.widget.handler.common.federatedSignIn(app, component,
              providerId, email);
        }
docs: On submit.