Attempt to reduce flakiness rate of io test

tests/standalone/io/socket_bind_test.dart

Index: tests/standalone/io/socket_bind_test.dart
diff --git a/tests/standalone/io/socket_bind_test.dart b/tests/standalone/io/socket_bind_test.dart
index f701f0c806109cad0db7dfbdad65763f8baa8a3a..9687eda69dfaec7becce75c76a83aa5b0f008ae7 100644
--- a/tests/standalone/io/socket_bind_test.dart
+++ b/tests/standalone/io/socket_bind_test.dart
@@ -4,11 +4,12 @@
 
 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(
@@ -112,14 +113,6 @@ testListenCloseListenClose(String host) async {
   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 {