Expose Zones.

tests/lib/async/zone_fork_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:expect/expect.dart';
+import 'package:async_helper/async_helper.dart';
+import 'dart:async';
+
+main() {
+  Completer done = new Completer();
+  List events = [];
+
+  Expect.identical(Zone.ROOT, Zone.current);
+  Zone forked;
+  forked = Zone.current.fork(specification: new ZoneSpecification(
+      fork: (Zone self, ZoneDelegate parent, Zone origin,
+             ZoneSpecification zoneSpecification, Map mapValues) {
+        // The zone is still the same as when origin.run was invoked, which
+        // is the root zone. (The origin zone hasn't been set yet).
+        Expect.identical(Zone.ROOT, Zone.current);
+        events.add("forked.fork");
+        Function descriptionRun = zoneSpecification.run;
+        ZoneSpecification modified = new ZoneSpecification.from(
+          zoneSpecification,
+          run: (self, parent, origin, f) {
+            events.add("wrapped run");
+            return descriptionRun(self, parent, origin, () {
+              events.add("wrapped f");
+              return f();
+            });
+          });
+        return parent.fork(origin, modified, mapValues);
+      }));
+
+  events.add("start");
+  Zone forkedChild = forked.fork(specification: new ZoneSpecification(
+      run: (Zone self, ZoneDelegate parent, Zone origin, f()) {
+        events.add("executing child run");
+        return parent.run(origin, f);
+      }));
+
+  events.add("after child fork");
+  Expect.identical(Zone.ROOT, Zone.current);
+
+  forkedChild.run(() {
+    events.add("child run");
+  });
+
+  events.add("after child run");
+
+  Expect.listEquals(
+    [ "start",
+      "forked.fork",
+      "after child fork",
+      "wrapped run", "executing child run", "wrapped f", "child run",
+      "after child run" ],
+    events);
+}