Yesterday's deadlock fix was wrong. It could drop notifications.

runtime/bin/dbg_message.cc

Index: runtime/bin/dbg_message.cc
diff --git a/runtime/bin/dbg_message.cc b/runtime/bin/dbg_message.cc
index 2a0e9056cbe8230b2aee9e99ec23455c083ac497..1d8c22c024ab9475b20c2c5b1a8ecdb81d2cff83 100644
--- a/runtime/bin/dbg_message.cc
+++ b/runtime/bin/dbg_message.cc
@@ -1097,17 +1097,11 @@ void DbgMsgQueue::MessageLoop() {
     // Handle all available vm service messages, up to a resume
     // request.
     bool resume = false;
-
-    // Release the message queue lock before handling service
-    // messages.  This allows notifications to come in while we are
-    // processing long requests and avoids deadlock with the PortMap
-    // lock in the vm.
-    msg_queue_lock_.Exit();
     while (!resume && Dart_HasServiceMessages()) {
+      msg_queue_lock_.Exit();
       resume = Dart_HandleServiceMessages();
+      msg_queue_lock_.Enter();
     }
-    msg_queue_lock_.Enter();
-
     if (resume) {
       break;
     }