fixes #221, initial sync*, async, async* implementation

test/codegen/language/asyncstar_concat_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 "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+
+range(start, end) async* {
+  for (int i = start; i < end; i++) {
+    yield i;
+  }
+}
+
+concat(a, b) async* {
+  yield* a;
+  yield* b;
+}
+
+test() async {
+  Expect.listEquals([1, 2, 3, 11, 12, 13],
+                    await concat(range(1, 4), range(11, 14)).toList());
+}
+
+main() {
+  asyncStart();
+  test().then((_) {
+    asyncEnd();
+  });
+}