Fixes shutdown crashes.

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 d0f13d783fce0fdc892cde9237790a333c6b3ed4..89bac102cff3fb2615f4ce33d95e032b4c7b5412 100644
--- a/runtime/bin/dbg_message.cc
+++ b/runtime/bin/dbg_message.cc
@@ -1409,21 +1409,22 @@ void DbgMsgQueueList::IsolateEventHandler(Dart_IsolateId isolate_id,
     msg_queue->SendIsolateEvent(isolate_id, kind);
     Dart_ExitScope();
   } else {
-    DebuggerConnectionHandler::WaitForConnection();
     DbgMsgQueue* msg_queue = GetIsolateMsgQueue(isolate_id);
-    ASSERT(msg_queue != NULL);
-    Dart_EnterScope();
-    msg_queue->SendQueuedMsgs();
-    msg_queue->SendIsolateEvent(isolate_id, kind);
-    if (kind == kInterrupted) {
-      msg_queue->MessageLoop();
-    } else {
-      ASSERT(kind == kShutdown);
-      RemoveIsolateMsgQueue(isolate_id);
-    }
-    Dart_ExitScope();
     // If there is no receive message queue, do not wait for a connection, and
     // ignore the message.
+    if (msg_queue != NULL) {

[zra, 2015/10/26 20:51:10]

The vm isolate does not have a debug message queue.

+      DebuggerConnectionHandler::WaitForConnection();
+      Dart_EnterScope();
+      msg_queue->SendQueuedMsgs();
+      msg_queue->SendIsolateEvent(isolate_id, kind);
+      if (kind == kInterrupted) {
+        msg_queue->MessageLoop();
+      } else {
+        ASSERT(kind == kShutdown);
+        RemoveIsolateMsgQueue(isolate_id);
+      }
+      Dart_ExitScope();
+    }
   }
 }