Ignore GC events in websocket_client_test

tests/standalone/vmservice/websocket_client_test.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.
 
 library unknown_command_test;
 
 import 'test_helper.dart';
 import 'package:expect/expect.dart';
 
 class ClientsRequestTest extends ServiceWebSocketRequestHelper {
   ClientsRequestTest(port) : super('ws://127.0.0.1:$port/ws');
   int _count = 0;
 
   onResponse(var seq, Map response) {
+    if (seq == null) {
+      // Ignore push events.
+      return;
+    }
     if (seq == 'cli') {
       // Verify response is correct for 'cli' sequence id.
       Expect.equals('ClientList', response['type']);
       Expect.equals(1, response['members'].length);
       _count++;
     } else if (seq == 'vm') {
       // Verify response is correct for 'vm' sequence id.
       Expect.equals('VM', response['type']);
       _count++;
     } else {
@@ skipping 17 matching lines @@
   var process = new TestLauncher('unknown_command_script.dart');
   process.launch().then((port) {
     new ClientsRequestTest(port).connect().then((test) {
       test.completed.then((_) {
         process.requestExit();
       });
       test.runTest();
     });
   });
 }