Adding more relative path support.

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("dart:io");
 
 void main() {
   testBaseFunctions();
   testCanonicalize();
   testJoinAppend();
+  testRelativeTo();
 }
 
 void testBaseFunctions() {
   testGetters(new Path("/foo/bar/fisk.hest"),
               ['/foo/bar', 'fisk.hest', 'fisk', 'hest'],
               'absolute canonical');
   testGetters(new Path(''),
               ['', '', '', ''],
               'empty');
   // This corner case leaves a trailing slash for the root.
@@ skipping 133 matching lines @@
   testAppend('/a/b', 'c', '/a/b/c');
   testAppend('a/b/', 'cd', 'a/b/cd');
   testAppend('.', '..', './..');
   testAppend('a/b', '/c/d', 'a/b//c/d');
   testAppend('', 'foo/bar', 'foo/bar');
   testAppend('/foo', '', '/foo/');
 
   // .join can only join a relative path to a path.
   // It cannot join an absolute path to a path.
   Expect.throws(() => new Path('/a/b/').join(new Path('/c/d')));
+}
 
-  // Test Path.relativeTo.
+void testRelativeTo() {
   Expect.equals('c/d',
                 new Path('/a/b/c/d').relativeTo(new Path('/a/b')).toString());
   Expect.equals('c/d',
                 new Path('/a/b/c/d').relativeTo(new Path('/a/b/')).toString());
   Expect.equals('.',
                 new Path('/a').relativeTo(new Path('/a')).toString());
 
+  // Trailing / in base path represents directory
+  Expect.equals('../../z/x/y',
+      new Path('/a/b/z/x/y').relativeTo(new Path('/a/b/c/d/')).toString());
+  Expect.equals('../z/x/y',
+      new Path('/a/b/z/x/y').relativeTo(new Path('/a/b/c/d')).toString());
+  Expect.equals('../z/x/y/',
+      new Path('/a/b/z/x/y/').relativeTo(new Path('/a/b/c/d')).toString());
+
+  Expect.equals('../../z/x/y',
+      new Path('/z/x/y').relativeTo(new Path('/a/b/c')).toString());
+  Expect.equals('../../../z/x/y',
+      new Path('/z/x/y').relativeTo(new Path('/a/b/c/')).toString());
+
   // Not implemented yet.  Should return new Path('../b/c/d/').
   Expect.throws(() =>
                 new Path('b/c/d/').relativeTo(new Path('a/')));
   // Should always throw - no relative path can be constructed.
   Expect.throws(() =>
                 new Path('a/b').relativeTo(new Path('../../d')));
 }