Move StartIsolateScope into MessageHandler::HandleMessages

runtime/vm/message_handler.cc

Index: runtime/vm/message_handler.cc
diff --git a/runtime/vm/message_handler.cc b/runtime/vm/message_handler.cc
index 741bbba44a3ab4d7d96b07b95e4560964848b575..4a5ad31615ca4e7cc7b1aadfd2da24e479ce5d0d 100644
--- a/runtime/vm/message_handler.cc
+++ b/runtime/vm/message_handler.cc
@@ -137,6 +137,9 @@ Message* MessageHandler::DequeueMessage(Message::Priority min_priority) {
 
 bool MessageHandler::HandleMessages(bool allow_normal_messages,
                                     bool allow_multiple_normal_messages) {
+  // If isolate() returns NULL StartIsolateScope does nothing.
+  StartIsolateScope start_isolate(isolate());
+
   // TODO(turnidge): Add assert that monitor_ is held here.
   bool result = true;
   Message::Priority min_priority = (allow_normal_messages && !paused()) ?