Make dart:_vmservice a proper builtin library

runtime/lib/vmservice/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;
@@ skipping 28 matching lines @@
   }
 
   // Sends a result to the client.  Implemented in subclasses.
   void post(dynamic result);
 
   dynamic toJson() {
     return {
     };
   }
 }