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

runtime/vm/service/client.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;
 
 // A service client.
 abstract class Client {
   final VMService service;
   final bool sendEvents;
 
   Client(this.service, { bool sendEvents: true })
       : this.sendEvents = sendEvents {
     service._addClient(this);
   }
 
   /// When implementing, call [close] when the network connection closes.
   void close() {
     service._removeClient(this);
   }
 
   /// Call to process a message. Response will be posted with 'seq'.
   void onMessage(var seq, Message message) {
     try {
       // Send message to service.
       service.route(message).then((response) {
         // Call post when the response arrives.
-        post(seq, response);
+        post(response);
       });
     } catch (e, st) {
       message.setErrorResponse('Internal error: $e');
-      post(seq, message.response);
+      post(message.response);
     }
   }
 
   // Sends a result to the client.  Implemented in subclasses.
-  void post(var seq, dynamic result);
+  void post(dynamic result);
 
   dynamic toJson() {
     return {
     };
   }
 }