Add InternalStyle:rootLength to implement isAbsolute and rootPrefix.

pkg/path/test/url_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.
 
 import 'package:unittest/unittest.dart';
 import 'package:path/path.dart' as path;
 
 main() {
   var context = new path.Context(style: path.Style.url,
       current: 'http://dartlang.org/root/path');
@@ skipping 19 matching lines @@
     expect(context.rootPrefix('a/b'), '');
     expect(context.rootPrefix('http://dartlang.org/a/c'),
         'http://dartlang.org');
     expect(context.rootPrefix('file:///a/c'), 'file://');
     expect(context.rootPrefix('/a/c'), '/');
     expect(context.rootPrefix('http://dartlang.org/'), 'http://dartlang.org');
     expect(context.rootPrefix('file:///'), 'file://');
     expect(context.rootPrefix('http://dartlang.org'), 'http://dartlang.org');
     expect(context.rootPrefix('file://'), 'file://');
     expect(context.rootPrefix('/'), '/');
+    expect(context.rootPrefix('foo/bar://'), '');
   });
 
   test('dirname', () {
     expect(context.dirname(''), '.');
     expect(context.dirname('a'), '.');
     expect(context.dirname('a/b'), 'a');
     expect(context.dirname('a/b/c'), 'a/b');
     expect(context.dirname('a/b.c'), 'a');
     expect(context.dirname('a/'), '.');
     expect(context.dirname('a/.'), 'a');
@@ skipping 715 matching lines @@
 
     test('with a root-relative URI', () {
       expect(context.prettyUri('/a/b'), '/a/b');
     });
 
     test('with a Uri object', () {
       expect(context.prettyUri(Uri.parse('a/b')), 'a/b');
     });
   });
 }