[dart:io] Don't close stdin with a socket finalizer

tests/standalone/io/stdio_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 stdin is *not* closed when an Isolate leaks it.
+
+import 'dart:async';
+import 'dart:io';
+import 'dart:isolate';
+
+import "package:async_helper/async_helper.dart";
+import "package:expect/expect.dart";
+
+void ConnectorIsolate(SendPort sendPort) {
+  stdin;
+  sendPort.send(true);
+}
+
+main() async {
+  asyncStart();
+  ReceivePort receivePort = new ReceivePort();
+  Isolate isolate = await Isolate.spawn(ConnectorIsolate, receivePort.sendPort);
+  Completer<Null> completer = new Completer<Null>();
+  receivePort.listen((msg) {
+    Expect.isTrue(msg is bool);
+    Expect.isTrue(msg);
+    isolate.kill();
+    completer.complete(null);
+  });
+  await completer.future;
+  stdin.listen((_) {}).cancel();
+  receivePort.close();
+  asyncEnd();
+}