Store zone in microtask entry.

sdk/lib/async/zone.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.
 
 part of dart.async;
 
 typedef dynamic ZoneCallback();
 typedef dynamic ZoneUnaryCallback(arg);
 typedef dynamic ZoneBinaryCallback(arg1, arg2);
 
@@ skipping 925 matching lines @@
   return f;
 }
 
 AsyncError _rootErrorCallback(Zone self, ZoneDelegate parent, Zone zone,
                               Object error, StackTrace stackTrace) => null;
 
 void _rootScheduleMicrotask(Zone self, ZoneDelegate parent, Zone zone, f()) {
   if (!identical(_ROOT_ZONE, zone)) {
     bool hasErrorHandler = !_ROOT_ZONE.inSameErrorZone(zone);
     f = zone.bindCallback(f, runGuarded: hasErrorHandler);
+    // Use root zone as event zone if the function is already bound.
+    zone = _ROOT_ZONE;
   }
-  _scheduleAsyncCallback(f);
+  _scheduleAsyncCallback(new _AsyncCallbackEntry(f, zone));
 }
 
 Timer _rootCreateTimer(Zone self, ZoneDelegate parent, Zone zone,
                        Duration duration, void callback()) {
   if (!identical(_ROOT_ZONE, zone)) {
     callback = zone.bindCallback(callback);
   }
   return Timer._createTimer(duration, callback);
 }
 
@@ skipping 286 matching lines @@
                                    handleUncaughtError: errorHandler);
   }
   Zone zone = Zone.current.fork(specification: zoneSpecification,
                                 zoneValues: zoneValues);
   if (onError != null) {
     return zone.runGuarded(body);
   } else {
     return zone.run(body);
   }
 }