force stream events delivery in dart

I am trying to test stream events in my service, and faced a problem: is any way to synchronously deliver events in async stream? Here is simplified code example:

test("deliver event", () {
  StreamController sc = new StreamController();
  String v = "old";
  sc.stream.listen((val) {v = val;});
  sc.add("new");
  expect(v, "new"); // test fails: actual value is "old"
});