When no service response is requested, send null back to the service isolate so it can cleanup

sdk/lib/developer/extension.dart

Index: sdk/lib/developer/extension.dart
diff --git a/sdk/lib/developer/extension.dart b/sdk/lib/developer/extension.dart
index 04b3149ccdae24594a5aebcfba4c3c36a41e7d39..f23b5ad5630b266f35c0ef1576cedf6595374ff8 100644
--- a/sdk/lib/developer/extension.dart
+++ b/sdk/lib/developer/extension.dart
@@ -158,9 +158,7 @@ _postResponse(SendPort replyPort,
   assert(replyPort != null);
   if (id == null) {
     // No id -> no response.
-    // TODO(johnmccutchan): This code and the code in service.cc leave the
-    // service isolate with an open port. Consider posting 'null' to indicate
-    // that no response is coming.
+    replyPort.send(null);
     return;
   }
   assert(id != null);