Metadata for a RequestController subclass that requires it must be instantiated for each request.
Requires that the RequestController must be created through RequestController.generate.
RequestControllers may carry some state throughout the course of their handling of a request. If that RequestController is reused for another request, some of that state may carry over. Therefore, it is a better solution to instantiate the RequestController for each incoming request. Marking a RequestController subclass with this flag will ensure that an exception is thrown if an instance of RequestController is chained in a RequestSink. These instances must be generated with a closure:
router.route("/path").generate(() => new RequestControllerSubclass());