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) {

[Cutch, 2015/10/26 21:39:44]

As discussed offline- this should remain an ASSERT because we shouldn't be
notifying the embedder that an isolate was shutdown when we never notified it
that it was created in the first place. We also don't want to leak information
about the vm-isolate to the embedder here.

[zra, 2015/10/26 21:49:58]

Done.

+      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();
+    }
   }
 }