Expose Zones.

tests/lib/async/zone_register_callback1_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();
+
+  var valueToCapture;
+  var restoredValue;
+
+  Expect.identical(Zone.ROOT, Zone.current);
+  Zone forked = Zone.current.fork(null, new ZoneDescription(
+      registerCallback1: (Zone self, ZoneDelegate parent, Zone origin, f(arg)) {
+        // 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.current, Zone.ROOT);
+        // Note that not forwarding is completely legal, though not encouraged.
+        var capturedValue = valueToCapture;
+        return parent.registerCallback1(origin, (arg) {
+          restoredValue = capturedValue;
+          return f(arg);
+        });
+      }));
+
+  valueToCapture = 499;
+  var fun = (x) => x + 99;
+  var registered = forked.registerCallback1(fun);
+  Expect.isFalse(identical(fun, registered));
+
+  // It is legal to invoke the callback in a different zone. This is, of course,

[Lasse Reichstein Nielsen, 2013/09/23 14:24:12]

I generally prefer "allowed" or "valid" to "legal" - reserving "legal" for
actual laws.

[floitsch, 2013/09/23 17:12:07]

Done.

+  // extremely discouraged.
+  var result = registered(399);
+  Expect.equals(498, result);
+  Expect.equals(499, restoredValue);
+}