Switch docs to path package (away from dart:io Path).

tools/dom/docs/bin/docs.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.
 
 /**
  * This program reads the HTML libraries from [LIB_PATH] and outputs their
  * documentation to [JSON_PATH].
  */
 
 import 'dart:io';
 import 'dart:async';
 
+import 'package:path/path.dart' as path;
+
 import '../lib/docs.dart';
 
-final Path json_path = scriptDir.append('../docs.json').canonicalize();
-final Path lib_path = scriptDir.append('../../../../sdk/').canonicalize();
+final String json_path = 
+    path.normalize(path.join(scriptDir, '..', 'docs.json'));
+final String lib_path = 
+    path.normalize(path.join(scriptDir, '..', '..', '..', '..', 'sdk'));
 
 main() {
   print('Converting HTML docs from $lib_path to $json_path.');
 
   convert(lib_path, json_path)
     .then((bool anyErrors) {
       print('Converted HTML docs ${anyErrors ? "with": "without"}'
         ' errors.');
     });
 }
 
 /**
  * Gets the full path to the directory containing the entrypoint of the current
  * script.
  */
-Path get scriptDir =>
-    new Path(new Options().script).directoryPath;
+String get scriptDir => path.dirname(new Options().script);