Expose Zones.

tests/lib/async/catch_errors4_test.dart

 // Copyright (c) 2013, 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:async_helper/async_helper.dart';
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'catch_errors.dart';
 
 main() {
   asyncStart();
+  Completer done = new Completer();
+
   var events = [];
   // Test that synchronous errors inside a `catchErrors` are caught.
   catchErrors(() {
     events.add("catch error entry");
     throw "catch error";
   }).listen((x) {
       events.add(x);
+      done.complete(true);
     },
-    onDone: () {
-      Expect.listEquals(["catch error entry",
-                         "main exit",
-                         "catch error",
-                         ],
-                         events);
-      asyncEnd();
-    });
+    onDone: () { Expect.fail("Unexpected callback"); });
+
+  done.future.whenComplete(() {
+    Expect.listEquals(["catch error entry",
+                        "main exit",
+                        "catch error",
+                        ],
+                        events);
+    asyncEnd();
+  });
   events.add("main exit");
 }