Add InternalStyle:rootLength to implement isAbsolute and rootPrefix.

pkg/path/lib/src/parsed_path.dart

 // Copyright (c) 2013, 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.parsed_path;
 
 import 'internal_style.dart';
 import 'style.dart';
 
 class ParsedPath {
@@ skipping 27 matching lines @@
   String get extension => _splitExtension()[1];
 
   /// `true` if this is an absolute path.
   bool get isAbsolute => root != null;
 
   factory ParsedPath.parse(String path, InternalStyle style) {
     var before = path;
 
     // Remove the root prefix, if any.
     var root = style.getRoot(path);
-    var isRootRelative = style.getRelativeRoot(path) != null;
+    var isRootRelative = style.isRootRelative(path);
     if (root != null) path = path.substring(root.length);
 
     // Split the parts on path separators.
     var parts = [];
     var separators = [];
 
     var start = 0;
 
     if (path.isNotEmpty && style.isSeparator(path.codeUnitAt(0))) {
       separators.add(path[0]);
@@ skipping 119 matching lines @@
     if (lastDot <= 0) return [file, ''];
 
     return [file.substring(0, lastDot), file.substring(lastDot)];
   }
 
   ParsedPath clone() => new ParsedPath._(
       style, root, isRootRelative,
       new List.from(parts), new List.from(separators));
 }