Attempt to reduce flakiness rate of io test

tests/standalone/io/socket_bind_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.
 
 import 'dart:async';
 import 'dart:io';
-import 'dart:convert';
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
+import 'test_utils.dart' show freeIPv4AndIPv6Port;
+
 testBindShared(String host, bool v6Only) {
   asyncStart();
   ServerSocket.bind(
       host, 0, v6Only: v6Only, shared: true).then((socket) {
     Expect.isTrue(socket.port > 0);
 
     asyncStart();
     return ServerSocket.bind(
         host, socket.port, v6Only: v6Only, shared: true).then((socket2) {
       Expect.equals(socket.address.address, socket2.address.address);
@@ skipping 83 matching lines @@
     asyncEnd();
   });
 
   Socket client = await Socket.connect(host, socket2.port);
   await client.close();
   await client.drain();
 
   asyncEnd();
 }
 
-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).");
   }
@@ skipping 23 matching lines @@
     negTestBindSharedMismatch(host, false);
     negTestBindSharedMismatch(host, true);
 
     negTestBindV6OnlyMismatch(host, true);
     negTestBindV6OnlyMismatch(host, false);
 
     testListenCloseListenClose(host);
   }
   asyncEnd();
 }