Expose Zones.

tests/lib/async/catch_errors22_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();
+
   var events = [];
   bool onDoneWasCalled = false;
   var controller = new StreamController();
   // Test that streams that are never closed keep the `catchError` alive.
   catchErrors(() {
     catchErrors(() {
       controller.stream.map((x) { throw x; }).listen((x) {
         // Should not happen.
         events.add("bad: $x");
       });
     }).listen((x) { events.add("caught: $x"); },
               onDone: () { events.add("done"); });
   }).listen((x) { events.add("outer: $x"); },
             onDone: () { onDoneWasCalled = true; });
 
   [1, 2, 3, 4].forEach(controller.add);
 
   new Timer(const Duration(milliseconds: 20), () {
     Expect.isFalse(onDoneWasCalled);
     Expect.listEquals(["caught: 1",
                        "caught: 2",
                        "caught: 3",
                        "caught: 4",
                       ],
                       events);
     asyncEnd();
   });
 }