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

runtime/bin/dbg_message.cc

 // Copyright (c) 2012, 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.
 
 #include "bin/dbg_connection.h"
 #include "bin/dbg_message.h"
 #include "bin/dartutils.h"
 #include "bin/lockers.h"
 #include "bin/thread.h"
 #include "bin/utils.h"
@@ skipping 1079 matching lines @@
   is_running_ = false;
 
   // Request notification on isolate messages.  This allows us to
   // respond to vm service messages while at breakpoint.
   Dart_SetMessageNotifyCallback(DbgMsgQueueList::NotifyIsolate);
 
   while (true) {
     // 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;
     }
 
     // Handle all available debug messages, up to a resume request.
     if (HandlePendingMessages()) {
       break;
     }
 
     // Wait for more debug or vm service messages.
@@ skipping 302 matching lines @@
     } else {
       ASSERT(kind == kShutdown);
       RemoveIsolateMsgQueue(isolate_id);
     }
   }
   Dart_ExitScope();
 }
 
 }  // namespace bin
 }  // namespace dart