Fix a flaky failure in socket_bind_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';
@@ skipping 45 matching lines @@
       Expect.isTrue('$error'.contains('v6Only flag'));
       socket.close().whenComplete(asyncEnd);
       asyncEnd();
     });
   });
 }
 
 Future testBindDifferentAddresses(InternetAddress addr1,
                                   InternetAddress addr2,
                                   bool addr1V6Only,
-                                  bool addr2V6Only) {
+                                  bool addr2V6Only) async {
+  int freePort = await freeIPv4AndIPv6Port();
+
   asyncStart();
   return ServerSocket.bind(
-      addr1, 0, v6Only: addr1V6Only, shared: false).then((socket) {
+      addr1, freePort, v6Only: addr1V6Only, shared: false).then((socket) {
     Expect.isTrue(socket.port > 0);
 
     asyncStart();
     return ServerSocket.bind(
-        addr2, socket.port, v6Only: addr2V6Only, shared: false).then((socket2) {
+        addr2, freePort, v6Only: addr2V6Only, shared: false).then((socket2) {
       Expect.equals(socket.port, socket2.port);
 
       return Future.wait([
           socket.close().whenComplete(asyncEnd),
           socket2.close().whenComplete(asyncEnd),
       ]);
     });
   });
 }
 
@@ skipping 18 matching lines @@
     asyncEnd();
   });
 
   Socket client = await Socket.connect(host, socket2.port);
   await client.close();
   await client.drain();
 
   asyncEnd();
 }
 
-void main() {
+Future<int> freeIPv4AndIPv6Port() async {
+  var socket = await ServerSocket.bind(InternetAddress.ANY_IP_V6, 0, v6Only: false);

[kustermann, 2016/05/26 16:56:27]

long line

+  int port = socket.port;
+  await socket.close();
+  return port;
+}
+
+void main() async {
+  asyncStart();
+  await testBindDifferentAddresses(InternetAddress.ANY_IP_V6,
+                                   InternetAddress.ANY_IP_V4,
+                                   true,
+                                   false);
+  await testBindDifferentAddresses(InternetAddress.ANY_IP_V4,
+                                   InternetAddress.ANY_IP_V6,
+                                   false,
+                                   true);
+
   for (var host in ['127.0.0.1', '::1']) {
     testBindShared(host, false);
     testBindShared(host, true);
 
     negTestBindSharedMismatch(host, false);
     negTestBindSharedMismatch(host, true);
 
     negTestBindV6OnlyMismatch(host, true);
     negTestBindV6OnlyMismatch(host, false);
 
     testListenCloseListenClose(host);
   }
-
-  asyncStart();
-  testBindDifferentAddresses(InternetAddress.ANY_IP_V6,
-                             InternetAddress.ANY_IP_V4,
-                             true,
-                             false).then((_) {
-    testBindDifferentAddresses(InternetAddress.ANY_IP_V4,
-                               InternetAddress.ANY_IP_V6,
-                               false,
-                               true);
-    asyncEnd();
-  });
+  asyncEnd();
 }