Fix flaky timeouts of socket_ipv6_test.

tests/standalone/io/test_utils.dart

 // Copyright (c) 2016, 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.
 
 import 'dart:async';
 import 'dart:io';
 
 Future<int> freeIPv4AndIPv6Port() async {
   var socket =
     await ServerSocket.bind(InternetAddress.ANY_IP_V6, 0, v6Only: false);
   int port = socket.port;
   await socket.close();
   return port;
 }
 
 Future retry(Future fun(), {int maxCount: 10}) async {
   for (int i = 0; i < maxCount; i++) {
     try {
       // If there is no exception this will simply return, otherwise we keep
       // trying.
       return await fun();
     } catch (_) {}
     print("Failed to execute test closure in attempt $i "
           "(${maxCount - i} retries left).");
   }
   return await fun();
 }
 
+
+retrySync(fun, {int maxCount: 10}) {
+  for (int i = 0; i < maxCount; i++) {
+    try {
+      // If there is no exception this will simply return, otherwise we keep
+      // trying.
+      return fun();
+    } catch (_) {}
+    print("Failed to execute test closure in attempt $i "
+          "(${maxCount - i} retries left).");
+  }
+  return fun();
+}
+