Make dart:_vmservice a proper builtin library

runtime/lib/vmservice/running_isolate.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 RunningIsolate implements MessageRouter {
   final int portId;
   final SendPort sendPort;
   final String name;
 
   RunningIsolate(this.portId, this.sendPort, this.name);
 
   String get serviceId => 'isolates/$portId';
 
   Future<String> route(Message message) {
     // Send message to isolate.
     return message.send(sendPort);
   }
 }