Clean up some warnings and deprecated calls.

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 100 matching lines @@
 /// Loads the user's OAuth2 credentials from the in-memory cache or the
 /// filesystem if possible. If the credentials can't be loaded for any reason,
 /// the returned [Future] will complete to null.
 Credentials _loadCredentials(SystemCache cache) {
   log.fine('Loading OAuth2 credentials.');
 
   try {
     if (_credentials != null) return _credentials;
 
     var path = _credentialsFile(cache);
-    if (!fileExists(path)) return;
+    if (!fileExists(path)) return null;
 
     var credentials = new Credentials.fromJson(readTextFile(path));
     if (credentials.isExpired && !credentials.canRefresh) {
       log.error("Pub's authorization to upload packages has expired and "
           "can't be automatically refreshed.");
       return null; // null means re-authorize.
     }
 
     return credentials;
   } catch (e) {
@@ skipping 64 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;
   });
 }