dart:io | Handle drive letters in Path.relativeTo on Windows.

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();
   testRaw();
   testCanonicalize();
   testJoinAppend();
   testRelativeTo();
   testWindowsShare();
+  testWindowsDrive();
 }
 
 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 268 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);
 }
+
+// Test that Windows drive information is handled correctly in relative
+// Path operations.
+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 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());
+}