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

tests/standalone/io/socket_finalizer_test.dart

+// Copyright (c) 2017, 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.
+//
+// This test checks that sockets belonging to an isolate are properly cleaned up
+// when the isolate shuts down abnormally. If the socket is not properly cleaned
+// up, the test will time out.
+
+import 'dart:async';
+import 'dart:io';
+import 'dart:isolate';
+
+import "package:async_helper/async_helper.dart";
+import "package:expect/expect.dart";
+
+ConnectorIsolate(int port) async {
+  Socket socket = await Socket.connect("127.0.0.1", port);
+  socket.listen((_) {});
+}
+
+main() async {
+  asyncStart();
+  ServerSocket server = await ServerSocket.bind("127.0.0.1", 0);
+  Isolate isolate = await Isolate.spawn(ConnectorIsolate, server.port);
+  Completer<Null> completer = new Completer<Null>();
+  server.listen((Socket socket) {
+    socket.listen((_) {}, onDone: () {
+      print("Socket closed normally");
+      completer.complete(null);
+      socket.close();
+    }, onError: (e) {
+      Expect.fail("Socket error $e");
+    });
+    isolate.kill();
+  });
+  await completer.future;
+  await server.close();
+  asyncEnd();
+}