code: function getComponents(nextState, callback) {
    mapAsync(nextState.routes, function (route, index, callback) {
      getComponentsForRoute(nextState, route, callback)
    }, callback)
  }
docs: 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.