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

runtime/bin/vmservice/server.dart

 // Copyright (c) 2013, 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 vmservice_io;
 
 class WebSocketClient extends Client {
   static const int PARSE_ERROR_CODE = 4000;
   static const int BINARY_MESSAGE_ERROR_CODE = 4001;
   static const int NOT_MAP_ERROR_CODE = 4002;
@@ skipping 22 matching lines @@
       if (serial != null && serial is! num && serial is! String) {
         socket.close(ID_ERROR_CODE, '"id" must be a number, string, or null.');
       }
       onMessage(serial, new Message.fromJsonRpc(this, map));
     } else {
       socket.close(BINARY_MESSAGE_ERROR_CODE, 'Message must be a string.');
     }
   }
 
   void post(dynamic result) {
+    if (result == null) {
+      // Do nothing.
+      return;
+    }
     try {
       socket.add(result);
     } catch (_) {
       print("Ignoring error posting over WebSocket.");
     }
   }
 
   dynamic toJson() {
     Map map = super.toJson();
     map['type'] = 'WebSocketClient';
     map['socket'] = '$socket';
     return map;
   }
 }
 
 
 class HttpRequestClient extends Client {
   static ContentType jsonContentType =
       new ContentType("application", "json", charset: "utf-8");
   final HttpRequest request;
 
   HttpRequestClient(this.request, VMService service)
       : super(service, sendEvents:false);
 
   void post(String result) {
+    if (result == null) {
+      close();
+      return;
+    }
     request.response..headers.contentType = jsonContentType
                     ..write(result)
                     ..close();
     close();
   }
 
   dynamic toJson() {
     Map map = super.toJson();
     map['type'] = 'HttpRequestClient';
     map['request'] = '$request';
@@ skipping 110 matching lines @@
       print('Could not shutdown Observatory HTTP server:\n$e\n$st\n');
       _notifyServerState("", 0);
       return this;
     });
   }
 
 }
 
 void _notifyServerState(String ip, int port)
     native "VMServiceIO_NotifyServerState";