Make path's top-level context field public.

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 23 matching lines @@
   });
 
   test('current', () {
     expect(path.current, io.Directory.current.path);
   });
 
   test('registers changes to the working directory', () {
     var dir = io.Directory.current.path;
     try {
       expect(path.absolute('foo/bar'), equals(path.join(dir, 'foo/bar')));
+      expect(path.absolute('foo/bar'),
+          equals(path.context.join(dir, 'foo/bar')));
 
       io.Directory.current = path.dirname(dir);
       expect(path.normalize(path.absolute('foo/bar')),
           equals(path.normalize(path.join(dir, '../foo/bar'))));
+      expect(path.normalize(path.absolute('foo/bar')),
+          equals(path.normalize(path.context.join(dir, '../foo/bar'))));
     } finally {
       io.Directory.current = dir;
     }
   });
 }