Add missing files from previous commit

sdk/lib/io/file_system_entity.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.
+
+part of dart.io;
+
+/**
+ * A [FileSystemEntity] is a common super class for [File] and
+ * [Directory] objects.
+ *
+ * [FileSystemEntity] objects are returned from directory listing
+ * operations. To determine if a FileSystemEntity is a File or a
+ * directory, perform a type check:
+ *
+ *    if (entity is File) (entity as File).readAsStringSync();
+ */
+abstract class FileSystemEntity {
+}