Add InternalStyle:rootLength to implement isAbsolute and rootPrefix.

pkg/path/test/windows_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.
 
 library path.test.windows_test;
 
 import 'package:unittest/unittest.dart';
 import 'package:path/path.dart' as path;
 
 import 'utils.dart';
@@ skipping 23 matching lines @@
   });
 
   test('rootPrefix', () {
     expect(context.rootPrefix(''), '');
     expect(context.rootPrefix('a'), '');
     expect(context.rootPrefix(r'a\b'), '');
     expect(context.rootPrefix(r'C:\a\c'), r'C:\');
     expect(context.rootPrefix('C:\\'), r'C:\');
     expect(context.rootPrefix('C:/'), 'C:/');
     expect(context.rootPrefix(r'\\server\share\a\b'), r'\\server\share');
+    expect(context.rootPrefix(r'\\server\share'), r'\\server\share');
+    expect(context.rootPrefix(r'\\server\'), r'\\server\');
+    expect(context.rootPrefix(r'\\server'), r'\\server');
     expect(context.rootPrefix(r'\a\b'), r'\');
     expect(context.rootPrefix(r'/a/b'), r'/');
     expect(context.rootPrefix(r'\'), r'\');
     expect(context.rootPrefix(r'/'), r'/');
   });
 
   test('dirname', () {
     expect(context.dirname(r''), '.');
     expect(context.dirname(r'a'), '.');
     expect(context.dirname(r'a\b'), 'a');
@@ skipping 610 matching lines @@
 
     test('with a root-relative URI', () {
       expect(context.prettyUri('/D:/a/b'), r'D:\a\b');
     });
 
     test('with a Uri object', () {
       expect(context.prettyUri(Uri.parse('a/b')), r'a\b');
     });
   });
 }