Add Path class to dart:io, and add unit tests for it.

runtime/bin/io_sources.gypi

 # Copyright (c) 2012, 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 file contains all sources for the dart:io library.
 {
   'sources': [
     # The io.dart file needs to be the first source file. It contains
     # the library and import directives for the dart:io library. The
     # dart:io library is created by concatenating the files listed here
@@ skipping 13 matching lines @@
     'http_impl.dart',
     'http_parser.dart',
     'http_utils.dart',
     'input_stream.dart',
     'list_stream.dart',
     'list_stream_impl.dart',
     'mime_multipart_parser.dart',
     'output_stream.dart',
     'stream_util.dart',
     'string_stream.dart',
+    'path.dart',
+    'path_impl.dart',
     'platform.dart',
     'platform_impl.dart',
     'process.dart',
     'process_impl.dart',
     'socket.dart',
     'socket_impl.dart',
     'socket_stream.dart',
     'socket_stream_impl.dart',
     'stdio.dart',
     'timer.dart',
     'timer_impl.dart',
     'websocket.dart',
     'websocket_impl.dart',
   ],
 }