Add InternalStyle:rootLength to implement isAbsolute and rootPrefix.

pkg/path/lib/src/style/posix.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.style.posix;
 
 import '../characters.dart' as chars;
 import '../parsed_path.dart';
 import '../internal_style.dart';
 
@@ skipping 12 matching lines @@
   final rootPattern = new RegExp(r'^/');
   final relativeRootPattern = null;
 
   bool containsSeparator(String path) => path.contains('/');
 
   bool isSeparator(int codeUnit) => codeUnit == chars.SLASH;
 
   bool needsSeparator(String path) =>
       path.isNotEmpty && !isSeparator(path.codeUnitAt(path.length - 1));
 
-  String getRoot(String path) {
-    if (path.isNotEmpty && isSeparator(path.codeUnitAt(0))) return '/';
-    return null;
+  int rootLength(String path) {
+    if (path.isNotEmpty && isSeparator(path.codeUnitAt(0))) return 1;
+    return 0;
   }
 
+  bool isRootRelative(String path) => false;
+
   String getRelativeRoot(String path) => null;
 
   String pathFromUri(Uri uri) {
     if (uri.scheme == '' || uri.scheme == 'file') {
       return Uri.decodeComponent(uri.path);
     }
     throw new ArgumentError("Uri $uri must have scheme 'file:'.");
   }
 
   Uri absolutePathToUri(String path) {
     var parsed = new ParsedPath.parse(path, this);
     if (parsed.parts.isEmpty) {
       // If the path is a bare root (e.g. "/"), [components] will
       // currently be empty. We add two empty components so the URL constructor
       // produces "file:///", with a trailing slash.
       parsed.parts.addAll(["", ""]);
     } else if (parsed.hasTrailingSeparator) {
       // If the path has a trailing slash, add a single empty component so the
       // URI has a trailing slash as well.
       parsed.parts.add("");
     }
 
     return new Uri(scheme: 'file', pathSegments: parsed.parts);
   }
 }