Attempt to reduce flakiness rate of io 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();
+}
+