Typecheck return with respect to async.

tests/language/sync_generator2_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Simple test program for sync* generator functions.
 
 import "package:expect/expect.dart";
 import "dart:async";
 
 var sync = "topLevelSync";
@@ skipping 37 matching lines @@
   sync() sync* {
     yield sync;  // Yields a tear-off of the sync() method.
   }
 }
 
 main() {
   var x;
   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);
 }