Add extra Path test.

tests/standalone/io/path_test.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.
 //
 // Test the Path class in dart:io.
 
 import "package:expect/expect.dart";
 import "dart:io";
 
 void main() {
@@ skipping 305 matching lines @@
 void testWindowsDrive() {
   // Windows drive information only makes sense on Windows.
   if (Platform.operatingSystem != 'windows') return;
   // Test that case of drive letters is ignored, and that drive letters
   // are treated specially.
   var CPath = new Path(r'C:\a\b\..\c');
   var cPath = new Path(r'c:\a\b\d');
   var C2Path = new Path(r'C:\a\b\d');
   var C3Path = new Path(r'C:\a\b');
   var C4Path = new Path(r'C:\');
+  var c4Path = new Path(r'C:\');

[Søren Gjesse, 2013/04/08 14:54:16]

Shouldn't it be a lower case c here???

[Anders Johnsen, 2013/04/08 14:55:58]

Hehe, oops :)

   var DPath = new Path(r'D:\a\b\d\e');
   var NoPath = new Path(r'\a\b\c\.');
 
   Expect.throws(() => CPath.relativeTo(DPath));
   Expect.throws(() => CPath.relativeTo(NoPath));
   Expect.throws(() => NoPath.relativeTo(CPath));
   Expect.equals('../../c', CPath.relativeTo(cPath).toString());
   Expect.equals('../b/d', cPath.relativeTo(CPath).toString());
   Expect.equals('.', DPath.relativeTo(DPath).toString());
   Expect.equals('.', NoPath.relativeTo(NoPath).toString());
   Expect.equals('.', C2Path.relativeTo(cPath).toString());
   Expect.equals('..', C3Path.relativeTo(cPath).toString());
   Expect.equals('d', cPath.relativeTo(C3Path).toString());
   Expect.equals('a/b/d', cPath.relativeTo(C4Path).toString());
   Expect.equals('../../../', C4Path.relativeTo(cPath).toString());
+  Expect.equals('a/b', C3Path.relativeTo(c4Path).toString());
 }