Get rid of join() and encapsulate File and Directory in io.dart.

utils/pub/oauth2.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.
 
 library oauth2;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:uri';
 
@@ skipping 129 matching lines @@
 void _saveCredentials(SystemCache cache, Credentials credentials) {
   log.fine('Saving OAuth2 credentials.');
   _credentials = credentials;
   var credentialsPath = _credentialsFile(cache);
   ensureDir(path.dirname(credentialsPath));
   writeTextFile(credentialsPath, credentials.toJson(), dontLogContents: true);
 }
 
 /// The path to the file in which the user's OAuth2 credentials are stored.
 String _credentialsFile(SystemCache cache) =>
-  join(cache.rootDir, 'credentials.json');
+    path.join(cache.rootDir, 'credentials.json');
 
 /// Gets the user to authorize pub as a client of pub.dartlang.org via oauth2.
 /// Returns a Future that will complete to a fully-authorized [Client].
 Future<Client> _authorize() {
   // Allow the tests to inject their own token endpoint URL.
   var tokenEndpoint = Platform.environment['_PUB_TEST_TOKEN_ENDPOINT'];
   if (tokenEndpoint != null) {
     tokenEndpoint = Uri.parse(tokenEndpoint);
   } else {
     tokenEndpoint = _tokenEndpoint;
@@ skipping 35 matching lines @@
       'Pub needs your authorization to upload packages on your behalf.\n'
       'In a web browser, go to $authUrl\n'
       'Then click "Allow access".\n\n'
       'Waiting for your authorization...');
 
   return completer.future.then((client) {
     log.message('Successfully authorized.\n');
     return client;
   });
 }