Do not JSON encode the 'result' of a service rpc.

runtime/vm/service/vmservice.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.
 
 library vmservice;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:isolate';
 import 'dart:typed_data';
@@ skipping 28 matching lines @@
     clients.add(client);
   }
 
   void _removeClient(Client client) {
     clients.remove(client);
   }
 
   void _eventMessageHandler(dynamic eventMessage) {
     for (var client in clients) {
       if (client.sendEvents) {
-        client.post(null, eventMessage);
+        client.post(eventMessage);
       }
     }
   }
 
   void _controlMessageHandler(int code,
                               int portId,
                               SendPort sp,
                               String name) {
     switch (code) {
       case Constants.ISOLATE_STARTUP_MESSAGE_ID:
@@ skipping 102 matching lines @@
 }
 
 void _registerIsolate(int port_id, SendPort sp, String name) {
   var service = new VMService();
   service.runningIsolates.isolateStartup(port_id, sp, name);
 }
 
 void _onStart() native "VMService_OnStart";
 
 void _onExit() native "VMService_OnExit";