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

tests/compiler/dart2js/type_checker_test.dart

Index: tests/compiler/dart2js/type_checker_test.dart
diff --git a/tests/compiler/dart2js/type_checker_test.dart b/tests/compiler/dart2js/type_checker_test.dart
index 0e6a54ea5b583fddaa7ee9249fdda9b81409c231..fc7b60127399c2af1af16c9f8dcbec35717b6c67 100644
--- a/tests/compiler/dart2js/type_checker_test.dart
+++ b/tests/compiler/dart2js/type_checker_test.dart
@@ -2385,6 +2385,8 @@ testAsyncReturn(MockCompiler compiler) {
     check("int foo() async => 0;", NOT_ASSIGNABLE),
     check("int foo() async => new Future<int>.value();",
           NOT_ASSIGNABLE),
+    check("Iterable<int> foo() sync* { return; }"),
+    check("Stream<int> foo() async* { return; }"),
   ]);
 }