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

sdk/lib/developer/extension.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.developer;
 
 class ServiceExtensionResponse {
   final String _result;
   final int _errorCode;
   final String _errorDetail;
@@ skipping 140 matching lines @@
   });
   return true;
 }
 
 _postResponse(SendPort replyPort,
               Object id,
               ServiceExtensionResponse response) {
   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);
   StringBuffer sb = new StringBuffer();
   sb.write('{"jsonrpc":"2.0",');
   if (response._isError()) {
     sb.write('"error":');
   } else {
     sb.write('"result":');
   }
   sb.write('${response._toString()},');
   if (id is String) {
     sb.write('"id":"$id"}');
   } else {
     sb.write('"id":$id}');
   }
   replyPort.send(sb.toString());
 }