Reland transformation of async functions.

tests/language/async_helper_lib.dart

Index: tests/language/async_helper_lib.dart
diff --git a/tests/language/deferred_constraints_lib2.dart b/tests/language/async_helper_lib.dart
similarity index 87%
copy from tests/language/deferred_constraints_lib2.dart
copy to tests/language/async_helper_lib.dart
index 830705f0cd4e96f3a5355dff33001eebc4b134fe..269ec6d7a71f9ce626534e844c75dd319c3a966b 100644
--- a/tests/language/deferred_constraints_lib2.dart
+++ b/tests/language/async_helper_lib.dart
@@ -2,6 +2,5 @@
 // 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.
 
-foo() => 42;
-
-class C {}
+library async;
+class async {}