_getHandlerForEngine(seen, routeName, localRouteName, engineInstance) {
    let handler = this._internalGetHandler(
      seen,
      routeName,
      localRouteName,
      engineInstance,
    );

    if (!hasDefaultSerialize(handler)) {
      throw new Error(
        'Defining a custom serialize method on an Engine route is not supported.',
      );
    }

    return handler;
  }
Gets the handler for a route from an Engine instance, proxies to the
_internalGetHandler method.

@private
@method _getHandlerForEngine
@param {Object} seen
@param {String} routeName
@param {String} localRouteName
@param {Owner} routeOwner
@return {EngineInstance} engineInstance