Implement scheduleImmediate for the VM.

sdk/lib/async/schedule_microtask.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>();
 
 void _asyncRunCallback() {
   // As long as we are iterating over the registered callbacks we don't
   // unset the [_callbacksAreEnqueued] boolean.
   while (!_asyncCallbacks.isEmpty) {
     Function callback = _asyncCallbacks.removeFirst();
     try {
       callback();
     } catch (e) {
-      _AsyncRun._scheduleImmediate(_asyncRunCallback);
+      scheduleImmediate(_asyncRunCallback);
       rethrow;
     }
   }
   // Any new callback must register a callback function now.
   _callbacksAreEnqueued = false;
 }
 
 void _scheduleAsyncCallback(callback) {
   // Optimizing a group of Timer.run callbacks to be executed in the
   // same Timer callback.
   _asyncCallbacks.add(callback);
   if (!_callbacksAreEnqueued) {
-    _AsyncRun._scheduleImmediate(_asyncRunCallback);
+    scheduleImmediate(_asyncRunCallback);
     _callbacksAreEnqueued = true;
   }
 }
 
 /**
  * Runs a function asynchronously.
  *
  * Callbacks registered through this function are always executed in order and
  * are guaranteed to run before other asynchronous events (like [Timer] events,
  * or DOM events).
@@ skipping 19 matching lines @@
 void scheduleMicrotask(void callback()) {
   if (Zone.current == Zone.ROOT) {
     // No need to bind the callback. We know that the root's scheduleMicrotask
     // will be invoked in the root zone.
     Zone.current.scheduleMicrotask(callback);
     return;
   }
   Zone.current.scheduleMicrotask(
       Zone.current.bindCallback(callback, runGuarded: true));
 }
-
-class _AsyncRun {
-  /** Schedule the given callback before any other event in the event-loop. */
-  external static void _scheduleImmediate(void callback());
-}