Fix bug in dart2js async rewrite

tests/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";
 
-range(start, end) async* {
-  for (int i = start; i < end; i++) {
-    yield i;
+class A {
+  var x = new B();
+
+  foo() async {
+    return x.foo == 2 ? 42 : x.foo;
   }
 }
 
-concat(a, b) async* {
-  yield* a;
-  yield* b;
-}
+class B {
+  var x = 0;
 
-test() async {
-  Expect.listEquals([1, 2, 3, 11, 12, 13],
-                    await concat(range(1, 4), range(11, 14)).toList());
+  get foo {
+    if (x == -1) {
+      return 0;
+    } else {
+      return x++;
+    }
+  }
 }
 
 main() {
   asyncStart();
-  test().then((_) {
+  new A().foo().then((result) {
+    Expect.equals(1, result);
     asyncEnd();
   });
 }