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

test/codegen/language/async_regression_23058_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.
+
+// Regression test for issue 23058.
+
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+
+class A {
+  var x = new B();
+
+  foo() async {
+    return x.foo == 2 ? 42 : x.foo;
+  }
+}
+
+class B {
+  var x = 0;
+
+  get foo {
+    if (x == -1) {
+      return 0;
+    } else {
+      return x++;
+    }
+  }
+}
+
+main() {
+  asyncStart();
+  new A().foo().then((result) {
+    Expect.equals(1, result);
+    asyncEnd();
+  });
+}