Fix deadlock that can occur while handling service messages at a breakpoint.

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 30eb7307216ce8beefcf2187106acf4f098582a3..e50754402223887f4360a5b6c438f3caf0c959ee 100644
--- a/runtime/bin/dbg_message.cc
+++ b/runtime/bin/dbg_message.cc
@@ -1096,8 +1096,18 @@ void DbgMsgQueue::MessageLoop() {
   while (true) {
     // Handle all available vm service messages, up to a resume
     // request.
-    if (Dart_HandleServiceMessages()) {
-      break;
+    while (Dart_HasServiceMessages()) {
+      // 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();
+      bool resume = Dart_HandleServiceMessages();
+      msg_queue_lock_.Enter();
+      if (resume) {
+        // Resume requested through the vm service.
+        break;
+      }
     }
 
     // Handle all available debug messages, up to a resume request.