Implement scheduleImmediate for the VM.

runtime/lib/schedule_immediate_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 */ void scheduleImmediate(void callback()) {
+  if (scheduleImmediateClosure == null) {
+    throw new UnsupportedError("microtasks are not supported");
+  }
+  scheduleImmediateClosure(callback);
+}
+
+typedef void ScheduleImmediateClosure(void callback());
+
+ScheduleImmediateClosure scheduleImmediateClosure;
+
+void setScheduleImmediateClosure(ScheduleImmediateClosure closure) {
+  scheduleImmediateClosure = closure;
+}