Implement async* functions in VM

tests/language/asyncstar_yield_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.
 
 import "dart:async";
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
 Stream<int> foo1() async* {
   yield 1;
@@ skipping 36 matching lines @@
 Stream<int> foo4() async* {
   int i = 0;
   try {
     while (true) {
       yield i;
       i++;
     }
   } finally {
     // Canceling the stream-subscription should run the finalizer.
     finalized.complete(true);
+    return;
   }
 }
 
 test() async {
   Expect.listEquals([1, 20], await (foo1().toList()));
   Expect.listEquals([0, 1, 2, 3], await (foo2().take(4).toList()));
   Expect.listEquals([null, -1, 0, 1, 2, 3, 0, 1, 2, 3],
       await (foo3(4).take(10).toList()));
   Expect.listEquals([0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
       await (foo4().take(10).toList()));
   Expect.isTrue(await (finalized.future));
 }
 
 main ()  {
   asyncStart();
   test().then((_) {
     asyncEnd();
   });
 }