Introduce and use Zone:_enter and Zone:_leave, in Future.

tests/lib/async/zone_run_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();
@@ skipping 44 matching lines @@
   Expect.equals(1234, result);
 
   done.future.whenComplete(() {
     Expect.listEquals(
         ["zone forked", "forked.run", "run closure", "executed run",
         "forked.run", "executed run2", "forked.run", "run closure 2",
         "after nested scheduleMicrotask", "forked.run", "run closure 3"],
         events);
     asyncEnd();
   });
+
+  var zone1 = Zone.ROOT.fork();
+  var zone2 = Zone.ROOT.fork();
+  var zone3 = Zone.ROOT.fork();
+  asyncStart();
+  asyncStart();
+  zone1.run(() {
+    var future = new Future.value();
+    zone2.run(() {
+      future.then((_) {
+        Expect.identical(zone2, Zone.current);
+        asyncEnd();
+      });
+    });
+    zone3.run(() {
+      future.then((_) {
+        Expect.identical(zone3, Zone.current);
+        asyncEnd();
+      });
+    });
+  });
 }