Don't double-close a WebSocket in web_socket_test.

test/web_socket_test.dart

 // Copyright (c) 2014, 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 http_parser.web_socket_test;
 
 import 'dart:io';
 
 import 'package:http_parser/http_parser.dart';
 import 'package:test/test.dart';
 
 void main() {
   test("a client can communicate with a WebSocket server", () {
     return HttpServer.bind("localhost", 0).then((server) {
       server.transform(new WebSocketTransformer()).listen((webSocket) {
         webSocket.add("hello!");
-        webSocket.first.then((request) {
+        webSocket.listen((request) {
           expect(request, equals("ping"));
           webSocket.add("pong");
           webSocket.close();
         });
       });
 
       var client = new HttpClient();
       return client
           .openUrl("GET", Uri.parse("http://localhost:${server.port}"))
           .then((request) {
@@ skipping 60 matching lines @@
             server.close();
           } else {
             fail("Only expected two messages.");
           }
           n++;
         }).asFuture();
       });
     });
   });
 }