Don't double-close a WebSocket in web_socket_test.

test/web_socket_test.dart

Index: test/web_socket_test.dart
diff --git a/test/web_socket_test.dart b/test/web_socket_test.dart
index 60934c5dd0be2e5c34a6a9152f344f10bc4daeab..4da985ac4399c7da539a4d17293235b8b2ec7c9a 100644
--- a/test/web_socket_test.dart
+++ b/test/web_socket_test.dart
@@ -14,7 +14,7 @@ void main() {
     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();