Add Dart_PostPriority function to dart_api, and use it in runAsync to schedule immediate events in …

runtime/lib/event_loop_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 _enqueueImmediate(void callback()) {
-    // TODO(9001): don't use the Timer to enqueue the immediate callback.
-    _createTimer(Duration.ZERO, callback);
+    if (_EnqueueImmediateFactory._factory == null) {
+      Timer.run(callback);
+      return;
+    }
+    _EnqueueImmediateFactory._factory(callback);
   }
 }
+
+typedef void _EnqueueImmediateFactoryClosure(void callback());
+
+class _EnqueueImmediateFactory {
+  static _EnqueueImmediateFactoryClosure _factory;
+}
+
+void _setEnqueueImmediateFactoryClosure(
+    _EnqueueImmediateFactoryClosure closure) {
+  _EnqueueImmediateFactory._factory = closure;
+}