Typecheck return with respect to async.

tests/language/sync_generator2_test.dart

Index: tests/language/sync_generator2_test.dart
diff --git a/tests/language/sync_generator2_test.dart b/tests/language/sync_generator2_test.dart
index 4e5aa08cc79daa95650f36d5350039d0305e32a2..4a0f9dd3aa1d65a3412e60979a68e7d58fe12c27 100644
--- a/tests/language/sync_generator2_test.dart
+++ b/tests/language/sync_generator2_test.dart
@@ -55,10 +55,13 @@ main() {
   x = test01();
   Expect.equals("()", x.toString());
   x = test02();
+  test03(); /// 30: continued
   Expect.equals("(12321)", x.toString());
   x = test04;  /// 40: continued
   test04 = x;  /// 41: continued
   x = new K();
+  print(x.garnix); /// 51: continued
+  x.etwas = null; /// 52: continued
   print(x.sync().toList());
   Expect.equals(1, x.sync().length);
 //  Expect.isTrue(x.sync().single is Function);