In a large heavy skillet heat the vegetable oil until it is hot but not smoking and in it stir fry the scallions, cabbage, and celery for 3 minutes, or until the cabbage is wilted. Add the sugar, broth, soy sauce, and sesame oil and simmer the mixture, covered, for 3 minutes. Stir the cornstarch mixture, stir it into the vegetable mixture, and bring the liquid to a boil. Stir in the pork and season to taste with red pepper flakes. Simmer until heated through and serve over noodles.