Update close_local_tests.dart

Author: brianquinlan

pkgs/web_socket_conformance_tests/lib/src/close_local_tests.dart

@@ -2,6 +2,7 @@
 // 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.
 
+import 'dart:isolate';
 import 'dart:typed_data';
 
 import 'package:async/async.dart';
@@ -135,7 +136,7 @@ void testCloseLocal(
         ..sendText('Hello World')
         ..sendText('Hello World 2')
         ..sendText('Hello World 3');
-      await channel.close(3000, 'Client initiated closure');
+      await Isolate.run(() => channel.close(3000, 'Client initiated closure'));
       final closeCode = await httpServerQueue.next as int?;
       final closeReason = await httpServerQueue.next as String?;