Implement scheduleImmediate for the VM.

sdk/lib/async/schedule_microtask.dart

Index: sdk/lib/async/schedule_microtask.dart
diff --git a/sdk/lib/async/schedule_microtask.dart b/sdk/lib/async/schedule_microtask.dart
index 94a68c5e772081edff84d9171a542114a4d3507b..e95aa22ec6e13e0f3934f59ba7d5af4cbc44fe31 100644
--- a/sdk/lib/async/schedule_microtask.dart
+++ b/sdk/lib/async/schedule_microtask.dart
@@ -17,7 +17,7 @@ void _asyncRunCallback() {
     try {
       callback();
     } catch (e) {
-      _AsyncRun._scheduleImmediate(_asyncRunCallback);
+      scheduleImmediate(_asyncRunCallback);
       rethrow;
     }
   }
@@ -30,7 +30,7 @@ void _scheduleAsyncCallback(callback) {
   // same Timer callback.
   _asyncCallbacks.add(callback);
   if (!_callbacksAreEnqueued) {
-    _AsyncRun._scheduleImmediate(_asyncRunCallback);
+    scheduleImmediate(_asyncRunCallback);
     _callbacksAreEnqueued = true;
   }
 }
@@ -70,8 +70,3 @@ void scheduleMicrotask(void callback()) {
   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());
-}