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

runtime/lib/event_loop_patch.dart

Index: runtime/lib/event_loop_patch.dart
diff --git a/runtime/lib/event_loop_patch.dart b/runtime/lib/event_loop_patch.dart
index fe04c060bed955d4c596ac7d6bd1a4ffd30eebe6..137c64b6646cf392744c2ac4246db80e7d2b5537 100644
--- a/runtime/lib/event_loop_patch.dart
+++ b/runtime/lib/event_loop_patch.dart
@@ -4,7 +4,21 @@
 
 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;
+}