Fix Path.relativeTo in special case where base is a substring of this.

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();
@@ skipping 218 matching lines @@
   Expect.equals('d',
       new Path('a/b/f/../c/d').relativeTo(new Path('a/e/../b/c')).toString());
   Expect.equals('..',
       new Path('a/b/f/../c').relativeTo(new Path('a/e/../b/c/e/')).toString());
   Expect.equals('../..',
       new Path('').relativeTo(new Path('a/b/')).toString());
   Expect.equals('../../..',
       new Path('..').relativeTo(new Path('a/b')).toString());
   Expect.equals('../b/c/d/',
       new Path('b/c/d/').relativeTo(new Path('a/')).toString());
+  Expect.equals('../a/b/c',
+      new Path('x/y/a//b/./f/../c').relativeTo(new Path('x//y/z')).toString());
+
+  // Case where base is a substring of relative:
+  Expect.equals('a/b',
+      new Path('/x/y//a/b').relativeTo(new Path('/x/y/')).toString());
+  Expect.equals('a/b',
+      new Path('x/y//a/b').relativeTo(new Path('x/y/')).toString());
+  Expect.equals('../ya/b',
+      new Path('/x/ya/b').relativeTo(new Path('/x/y')).toString());
+  Expect.equals('../ya/b',
+      new Path('x/ya/b').relativeTo(new Path('x/y')).toString());
+  Expect.equals('../b',
+      new Path('x/y/../b').relativeTo(new Path('x/y/.')).toString());
+  Expect.equals('a/b/c',
+      new Path('x/y/a//b/./f/../c').relativeTo(new Path('x/y')).toString());
+  Expect.equals('.',
+      new Path('/x/y//').relativeTo(new Path('/x/y/')).toString());
+  Expect.equals('.',
+      new Path('/x/y/').relativeTo(new Path('/x/y')).toString());
 
   // Should always throw - no relative path can be constructed.
   Expect.throws(() =>
                 new Path('a/b').relativeTo(new Path('../../d')));
   // Should always throw - relative and absolute paths are compared.
   Expect.throws(() =>
                 new Path('/a/b').relativeTo(new Path('c/d')));
 
   Expect.throws(() =>
                 new Path('a/b').relativeTo(new Path('/a/b')));
@@ skipping 25 matching lines @@
   Expect.isTrue(canonical.toString().startsWith('/share/a'));
   Expect.isTrue(canonical.toNativePath().startsWith(r'\\share\a'));
   Expect.listEquals(['share', 'a', 'c'], canonical.segments());
   var appended = canonical.append('d');
   Expect.isTrue(appended.isAbsolute);
   Expect.isTrue(appended.isWindowsShare);
   var directoryPath = canonical.directoryPath;
   Expect.isTrue(directoryPath.isAbsolute);
   Expect.isTrue(directoryPath.isWindowsShare);
 }