getBeforeSignInSuccessCallback() {
    return /** @type {?function(!firebase.User)} */ (
        this.getCallbacks_()['beforeSignInSuccess'] || null);
  }
Returns the beforeSignInSuccess callback. It is triggered before the user
signs in successfully. The user signing in is passed.
@return {?function(!firebase.User)} The beforeSignInSuccess callback,
    null if not available.