Attempt to reduce flakiness rate of io test

tests/standalone/io/test_utils.dart

Index: tests/standalone/io/test_utils.dart
diff --git a/tests/standalone/io/test_utils.dart b/tests/standalone/io/test_utils.dart
new file mode 100644
index 0000000000000000000000000000000000000000..ad0d71ccb94e60d235129f795e953ae6bdea9272
--- /dev/null
+++ b/tests/standalone/io/test_utils.dart
@@ -0,0 +1,14 @@
+// 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;
+}