Have pkg/path track the current working directory if it changes.

pkg/path/test/io_test.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.
 
 import 'dart:io' as io;
 
 import 'package:unittest/unittest.dart';
 import 'package:path/path.dart' as path;
 
 main() {
@@ skipping 18 matching lines @@
     if (io.Platform.operatingSystem == 'windows') {
       expect(path.Style.platform, path.Style.windows);
     } else {
       expect(path.Style.platform, path.Style.posix);
     }
   });
 
   test('current', () {
     expect(path.current, io.Directory.current.path);
   });
+
+  test('registers changes to the working directory', () {
+    expect(path.absolute('foo/bar'),
+        equals(path.join(io.Directory.current.path, 'foo/bar')));
+
+    var sandbox = io.Directory.systemTemp.createTempSync('path_test_').path;
+    try {
+      io.Directory.current = sandbox;
+
+      expect(path.absolute('foo/bar'), equals(path.join(sandbox, 'foo/bar')));
+    } finally {
+      new io.Directory(sandbox).deleteSync(recursive: true);
+    }
+  });
 }