Refactor Future constructors.

pkg/oauth2/lib/src/client.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_client;
 
 import 'dart:async';
 import 'dart:uri';
 
 import 'package:http/http.dart' as http;
@@ skipping 65 matching lines @@
       this.secret,
       this._credentials,
       {http.Client httpClient})
     : _httpClient = httpClient == null ? new http.Client() : httpClient;
 
   /// Sends an HTTP request with OAuth2 authorization credentials attached. This
   /// will also automatically refresh this client's [Credentials] before sending
   /// the request if necessary.
   Future<http.StreamedResponse> send(http.BaseRequest request) {
     return async.then((_) {
-      if (!credentials.isExpired) return new Future.immediate(null);
+      if (!credentials.isExpired) return new Future.value();
       if (!credentials.canRefresh) throw new ExpirationException(credentials);
       return refreshCredentials();
     }).then((_) {
       request.headers['authorization'] = "Bearer ${credentials.accessToken}";
       return _httpClient.send(request);
     }).then((response) {
       if (response.statusCode != 401 ||
           !response.headers.containsKey('www-authenticate')) {
         return response;
       }
@@ skipping 40 matching lines @@
       return this;
     });
   }
 
   /// Closes this client and its underlying HTTP client.
   void close() {
     if (_httpClient != null) _httpClient.close();
     _httpClient = null;
   }
 }