Remove usage of dart:json.

runtime/bin/vmservice/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:json' as JSON;
+import "dart:convert";
 import 'dart:isolate';
 import 'dart:typed_data';
 
 part 'constants.dart';
 part 'resources.dart';
 part 'running_isolate.dart';
 part 'running_isolates.dart';
 part 'service_request.dart';
 part 'service_request_router.dart';
 
@@ skipping 25 matching lines @@
   factory VMService() {
     if (VMService._instance == null) {
       VMService._instance = new VMService._internal();
     }
     return _instance;
   }
 }
 
 void sendServiceMessage(SendPort sp, ReceivePort rp, Object m)
     native "SendServiceMessage";