[dart:io] Adds a finalizer to _NativeSocket to avoid socket leaks

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 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
@@ skipping 92 matching lines @@
   socket2.first.then((socket) async {
     await socket.drain();
     await socket.close();
     asyncEnd();
   });
 
   Socket client = await Socket.connect(host, socket2.port);
   await client.close();
   await client.drain();
 
+  // Close the second server socket.
+  await socket2.close();
   asyncEnd();
 }
 
 main() async {
   asyncStart();
 
   await retry(() async {
     await testBindDifferentAddresses(InternetAddress.ANY_IP_V6,
                                      InternetAddress.ANY_IP_V4,
                                      true,
@@ skipping 13 matching lines @@
     negTestBindSharedMismatch(host, false);
     negTestBindSharedMismatch(host, true);
 
     negTestBindV6OnlyMismatch(host, true);
     negTestBindV6OnlyMismatch(host, false);
 
     testListenCloseListenClose(host);
   }
   asyncEnd();
 }