Update of language/async_star/throw_in_catch_test to correct async* semantics caused dart2js failure.

[mkustermann]

After changing language/async_star/throw_in_catch_test in cl/239421 to the semantics we want for async* (verified with lrn@), dart2js fails this test.

/cc @rakudrama

[mkustermann]

Small reproduction that demonstrates the issue

foo() async* {
  print('a');
  yield 1;
  print('should not print (our listener cancelled)');
}

main() async {
  var subscription;
  subscription = foo().listen((event) {
    subscription.cancel();
  });
}