2nd attempt at adding streamListen/streamCancel to the service protocol.

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;
+  final Set<String> streams = new Set<String>();
 
   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);
   }
@@ skipping 13 matching lines @@
   }
 
   // Sends a result to the client.  Implemented in subclasses.
   void post(dynamic result);
 
   dynamic toJson() {
     return {
     };
   }
 }