function getComponents(nextState, callback) {
	  (0, _AsyncUtils.mapAsync)(nextState.routes, function (route, index, callback) {
	    getComponentsForRoute(nextState, route, callback);
	  }, callback);
	}
Asynchronously fetches all components needed for the given router
state and calls callback(error, components) when finished.

Note: This operation may finish synchronously if no routes have an
asynchronous getComponents method.