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

lib/compiler/implementation/lib/io.dart

 // 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 is a copy of the VM's dart:io library. This API is not usable
 // when running inside a web browser. Nevertheless, Leg provides a
 // mock version of the dart:io library so that it can statically
 // analyze programs that use dart:io.
 
 // TODO(ahe): Separate API from implementation details.
@@ skipping 18 matching lines @@
 // Uses native keyword.
 // #source('../../../../runtime/bin/file_impl.dart');
 #source('../../../../runtime/bin/http.dart');
 #source('../../../../runtime/bin/http_impl.dart');
 #source('../../../../runtime/bin/http_parser.dart');
 #source('../../../../runtime/bin/http_utils.dart');
 #source('../../../../runtime/bin/input_stream.dart');
 #source('../../../../runtime/bin/list_stream.dart');
 #source('../../../../runtime/bin/list_stream_impl.dart');
 #source('../../../../runtime/bin/output_stream.dart');
-#source('../../../../runtime/bin/stream_util.dart');
-#source('../../../../runtime/bin/string_stream.dart');
+#source('../../../../runtime/bin/path.dart');
+#source('../../../../runtime/bin/path_impl.dart');
 #source('../../../../runtime/bin/platform.dart');
 // Uses native keyword.
 // #source('../../../../runtime/bin/platform_impl.dart');
 #source('../../../../runtime/bin/process.dart');
 // Uses native keyword.
 // #source('../../../../runtime/bin/process_impl.dart');
 #source('../../../../runtime/bin/socket.dart');
 // Uses native keyword.
 // #source('../../../../runtime/bin/socket_impl.dart');
 #source('../../../../runtime/bin/socket_stream.dart');
 #source('../../../../runtime/bin/socket_stream_impl.dart');
 // Uses native keyword.
 // #source('../../../../runtime/bin/stdio.dart');
+#source('../../../../runtime/bin/stream_util.dart');
+#source('../../../../runtime/bin/string_stream.dart');
 #source('../../../../runtime/bin/timer.dart');
 #source('../../../../runtime/bin/timer_impl.dart');
 #source('../../../../runtime/bin/websocket.dart');
 #source('../../../../runtime/bin/websocket_impl.dart');
 
 class _File {
   factory File(arg) {
     throw new UnsupportedOperationException('new File($arg)');
   }
 }
@@ skipping 83 matching lines @@
   static void set _eventHandler(_EventHandler e) {
     throw new UnsupportedOperationException('_EventHandler._eventhandler = $e');
   }
 }
 
 final InputStream stdin = null;
 
 final OutputStream stdout = null;
 
 final OutputStream stderr = null;