Use path package in dartdoc

sdk/lib/_internal/dartdoc/test/dartdoc_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.
 
 /// Unit tests for doc.
 library dartdocTests;
 
 import 'dart:async';
 import 'dart:io';
 
@@ skipping 220 matching lines @@
 /// ProcessResult.
 Future<ProcessResult> _runDartdoc(List<String> libraryPaths) {
   var dartBin = Platform.executable;
 
   var dartdoc = path.join(_dartdocDir, 'bin/dartdoc.dart');
 
   final runArgs = ['--package-root=$_packageRoot/', dartdoc];
 
   // Turn relative libraryPaths to absolute ones.
   runArgs.addAll(libraryPaths
-      .map((e) => path.join(dd.scriptDir.toNativePath(), e)));
+      .map((e) => path.join(path.absolute(dd.scriptDir), e)));
 
   return Process.run(dartBin, runArgs);
 }
 
 final _dartdocCompletionRegExp =
   new RegExp(r'Documentation complete -- documented (\d+) libraries, (\d+) types, and (\d+) members\.');
 
 void _expectDocumented(String output, { int libCount, int typeCount,
   int memberCount}) {
 
@@ skipping 21 matching lines @@
   }
 }
 
 
 validateDartdocMarkdown(String description, String markdown,
     String html) {
   var dartdoc = new dd.Dartdoc();
   validate(description, markdown, html, linkResolver: dartdoc.dartdocResolver,
       inlineSyntaxes: dartdoc.dartdocSyntaxes);
 }