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

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
diff --git a/runtime/include/dart_api.h b/runtime/include/dart_api.h
index 9f4969e16f5095a1359b46da028f9eda40fa91e2..e85291b3c43462849d0d725efcb67c56cc7cd5dd 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -1012,6 +1012,19 @@ DART_EXPORT bool Dart_HasLivePorts();
 DART_EXPORT bool Dart_Post(Dart_Port port_id, Dart_Handle object);
 
 /**
+ * Posts a message for some isolate. The message is a serialized
+ * object. The message is prioritized before other messages.
+ *
+ * Requires there to be a current isolate.
+ *
+ * \param port The destination port.
+ * \param object An object from the current isolate.
+ *
+ * \return True if the message was posted.
+ */
+DART_EXPORT bool Dart_PostPriority(Dart_Port port_id, Dart_Handle object);
+
+/**
  * Returns a new SendPort with the provided port id.
  */
 DART_EXPORT Dart_Handle Dart_NewSendPort(Dart_Port port_id);