Fix Path.relativeTo and add support for relative paths as input.

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 165 matching lines @@
   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')));
 }
 
 void testRelativeTo() {
+  // Cases where the arguments are absolute paths.
   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
+  // Trailing slash in the base path has no effect.  This matches Path.join
+  // semantics, but not URL join semantics.
   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',
+  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/',
+  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',
+  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/')));
+  // Cases where the arguments are relative paths.
+  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/b/c').relativeTo(new Path('a/b/c')).toString());
+  Expect.equals('.',
+      new Path('').relativeTo(new Path('')).toString());
+  Expect.equals('.',
+      new Path('.').relativeTo(new Path('.')).toString());
+  Expect.equals('a',
+      new Path('a').relativeTo(new Path('.')).toString());
+  Expect.equals('..',
+      new Path('..').relativeTo(new Path('.')).toString());
+  Expect.equals('..',
+      new Path('.').relativeTo(new Path('a')).toString());
+  Expect.equals('.',
+      new Path('..').relativeTo(new Path('..')).toString());
+  Expect.equals('./',
+      new Path('a/b/f/../c/').relativeTo(new Path('a/e/../b/c')).toString());
+  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());
+
   // 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')));
+
 }
 
-// Test that Windows share information is maintain through
+// Test that Windows share information is maintained through
 // Path operations.
 void testWindowsShare() {
   // Windows share information only makes sense on Windows.
   if (Platform.operatingSystem != 'windows') return;
   var path = new Path(r'\\share\a\b\..\c');
   Expect.isTrue(path.isAbsolute);
   Expect.isTrue(path.isWindowsShare);
   Expect.isFalse(path.hasTrailingSeparator);
   var canonical = path.canonicalize();
   Expect.isTrue(canonical.isAbsolute);
@@ skipping 11 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);
 }