Remove usage of dart:json.

runtime/bin/vmservice/service_request.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;
 
 class ServiceRequest {
   final List<String> pathSegments = new List<String>();
   final Map<String, String> parameters = new Map<String, String>();
   String _response;
@@ skipping 18 matching lines @@
       parameters[k] = v;
     });
     return true;
   }
 
   List toServiceCallMessage() {
     return [pathSegments, parameters.keys.toList(), parameters.values.toList()];
   }
 
   void setErrorResponse(String error) {
-    _response = JSON.stringify({
+    _response = JSON.encode({
         'type': 'error',
         'msg': error,
         'pathSegments': pathSegments,
         'parameters': parameters
     });
   }
 
   void setResponse(String response) {
     _response = response;
   }
 
 }