Avoid warning when using `return;` from sync* and async* functions.

tests/language/async_return_types_test.dart

Index: tests/language/async_return_types_test.dart
diff --git a/tests/language/async_return_types_test.dart b/tests/language/async_return_types_test.dart
index 83eb9a8d992f441ece1e655ef2cece1056798739..1134e25b0038c1d9caf25cac9229d5972427410d 100644
--- a/tests/language/async_return_types_test.dart
+++ b/tests/language/async_return_types_test.dart
@@ -40,6 +40,23 @@ foo7() async {
   return new Future<int>.value(3);
 }
 
+
+Iterable<int> foo8() sync* {
+  yield 1;
+  // Can only have valueless return in sync* functions.
+  return
+      8 /// return_value_sync_star: compile-time error
+       ;
+}
+
+Stream<int> foo9() async* {
+  yield 1;
+  // Can only have valueless return in async* functions.
+  return
+      8 /// return_value_sync_star: compile-time error
+       ;
+}
+
 test() async {
   Expect.equals(3, await foo1());
   Expect.equals(3, await foo2());
@@ -48,6 +65,8 @@ test() async {
   Expect.equals(3, await foo5());
   Expect.equals(3, await await foo6());
   Expect.equals(3, await await foo7());
+  Expect.listEquals([1], foo8().toList());
+  Expect.listEquals([1], await foo9().toList());
 }
 
 main() {