Fix a flaky failure in socket_bind_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 9c4e1ed284e29db1cf177d5679a1a772e96c1210..c764f6ec40ce42b41d7d2017a7f7feaf061f1ac8 100644
--- a/tests/standalone/io/socket_bind_test.dart
+++ b/tests/standalone/io/socket_bind_test.dart
@@ -63,15 +63,17 @@ negTestBindV6OnlyMismatch(String host, bool v6Only) {
 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([
@@ -110,7 +112,24 @@ testListenCloseListenClose(String host) async {
   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);
@@ -123,16 +142,5 @@ void main() {
 
     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();
 }