move proxy out into sample/proxy

samples/proxy/proxy.dart

 // Copyright (c) 2011, 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("proxy");
+
 /**
  * Base class for all RpcProxy's
  *
  * RpcProxy objects run in the "client" isolate and have a SendPort which
  * they use to send messages to RpcReceiver objects running in a "service"
  * isolate.
  */
 
 class RpcProxy {
   final Future<SendPort> futurePort;
@@ skipping 213 matching lines @@
     if (object === null || !(object is String)) {
       return null;
     }
     String s = object;
     if (!s.startsWith(prefix)) {
       return null;
     }
     return new RpcException(s.substring(prefix.length, s.length));
   }
 }