class ActionDispatch::Routing::RouteSet::Dispatcher

def call(env)
  params = env[PARAMETERS_KEY]
  prepare_params!(params)
  # Just raise undefined constant errors if a controller was specified as default.
  unless controller = controller(params, @defaults.key?(:controller))
    return [404, {'X-Cascade' => 'pass'}, []]
  end
  dispatch(controller, params[:action], env)
end