How to unit test Stream.listen() in Dart

Question

Consider the following test code ...

main() {
  StreamController controller;
  setUp(() {
    controller = StreamController.broadcast(sync: false);
  });

  tearDown(() {
    controller.close();
  });

  test('Stream listen test', () {
    var stream = controller.stream;

    stream.listen((event) {
      // >>>>> How can I test if this was called?? <<<<<<
    });

    controller.add('Here is an event!');
  });
}

... what is the correct way to test that the stream.listen(...) call-back was invoked?

stream.listen(...) doesn't return a Future so traditional expectAsyncX style matchers will not work here.

Perhaps there is some way to use the StreamSubscription.asFuture() that is returned by stream.listen(...)?