Reland transformation of async functions.

tests/language/async_backwards_compatibility_1_test.dart

 // Copyright (c) 2014, 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.
 
+import 'dart:async';
+import 'async_helper_lib.dart' as async;
+
 class A {
-  static const S = 'A.S';
+  async.async get async => null;
 }
 
-const S = 'S';
-
-foo(var p) {
-  switch (p) {
-    case S:
-      break;
-    case A.S:
-      break;
-    case 'abc':
-      break;
-  }
-}
+async.async topLevel() => null;
 
 main() {
-  foo('p');
+  var a = new A();
+  var b = a.async;
+  var c = topLevel();
 }