Expose Zones.

tests/lib/async/catch_errors28_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 periodic Timers are handled correctly by `catchErrors`.
   catchErrors(() {
     int counter = 0;
     new Timer.periodic(const Duration(milliseconds: 50), (timer) {
-      if (counter == 5) timer.cancel();
+      if (counter == 5) {
+        timer.cancel();
+        done.complete(true);
+      }
       counter++;
       events.add(counter);
     });
   }).listen((x) {
       events.add(x);
     },
-    onDone: () {
+    onDone: () { Expect.fail("Unexpected callback"); });
+
+  done.future.whenComplete(() {
+    // Give handlers time to run.
+    Timer.run(() {
       Expect.listEquals([
                          "main exit",
                          1, 2, 3, 4, 5, 6,
                          ],
                          events);
       asyncEnd();
     });
+  });
+
   events.add("main exit");
 }