Implement scheduleImmediate for the VM.

runtime/lib/schedule_microtask_patch.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.
 
 patch class _AsyncRun {
   /* patch */ static void _scheduleImmediate(void callback()) {
     if (_ScheduleImmediate._closure == null) {
-      // TODO(9001): don't default to using the Timer to enqueue the immediate
-      //             callback.
-      _createTimer(Duration.ZERO, callback);
-      return;
+      throw new UnsupportedError("Microtasks are not supported");
     }
     _ScheduleImmediate._closure(callback);
   }
 }
 
 typedef void _ScheduleImmediateClosure(void callback());
 
 class _ScheduleImmediate {
   static _ScheduleImmediateClosure _closure;
 }
 
 void _setScheduleImmediateClosure(_ScheduleImmediateClosure closure) {
   _ScheduleImmediate._closure = closure;
 }