Expose Zones.

sdk/lib/async/event_loop.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 void _AsyncCallback();
 
 bool _callbacksAreEnqueued = false;
 Queue<_AsyncCallback> _asyncCallbacks = new Queue<_AsyncCallback>();
@@ skipping 37 matching lines @@
  *
  *     Timer.run(() { print("executed"); });  // Will never be executed;
  *     foo() {
  *       asyncRun(foo);  // Schedules [foo] in front of other events.
  *     }
  *     main() {
  *       foo();
  *     }
  */
 void runAsync(void callback()) {
-  _Zone currentZone = _Zone._current;
-  currentZone.runAsync(callback, currentZone);
+  if (Zone.current == Zone.ROOT) {
+    // No need to bind the callback. We know that the root's runAsync will
+    // be invoked in the root zone.
+    Zone.current.scheduleMicrotask(callback);
+    return;
+  }
+  Zone.current.scheduleMicrotask(
+      Zone.current.bindCallback(callback, runGuarded: true));
 }
 
 class _AsyncRun {
   /** Enqueues the given callback before any other event in the event-loop. */
   external static void _enqueueImmediate(void callback());
 }