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

runtime/vm/message.cc

Index: runtime/vm/message.cc
diff --git a/runtime/vm/message.cc b/runtime/vm/message.cc
index 73f5da3c76b62652c2ee2c1b52285831e59524c6..7df75a3c18c8b3f9c1f50c016c75b4edcf19fe7a 100644
--- a/runtime/vm/message.cc
+++ b/runtime/vm/message.cc
@@ -29,9 +29,15 @@ void MessageQueue::Enqueue(Message* msg) {
     tail_ = msg;
   } else {
     ASSERT(tail_ != NULL);
-    // Append at the tail.
-    tail_->next_ = msg;
-    tail_ = msg;
+    if (msg->priority() == Message::kFirstPriority) {
+      // Set as head element in the queue.
+      msg->next_ = head_;
+      head_ = msg;
+    } else {
+      // Append at the tail.
+      tail_->next_ = msg;
+      tail_ = msg;
+    }
   }
 }