Implement scheduleImmediate for the VM.

runtime/lib/schedule_microtask_patch.dart

Index: runtime/lib/schedule_microtask_patch.dart
diff --git a/runtime/lib/schedule_microtask_patch.dart b/runtime/lib/schedule_microtask_patch.dart
index 75e87c8eb2fde52d0e0fa2e426e0a8478f0f2960..f08de2b907908206f03ae5fe21fd215cf64ddac4 100644
--- a/runtime/lib/schedule_microtask_patch.dart
+++ b/runtime/lib/schedule_microtask_patch.dart
@@ -5,10 +5,7 @@
 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);
   }