Change Future.delayed to take a Duration.

pkg/oauth2/lib/src/utils.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 utils;
 
 import 'dart:async';
 import 'dart:uri';
 import 'dart:isolate';
 import 'dart:crypto';
@@ skipping 95 matching lines @@
 
     if (!paramString.trim().isEmpty) {
       throw new FormatException('Invalid WWW-Authenticate header: "$header"');
     }
 
     return new AuthenticateHeader(scheme, parameters);
   }
 }
 
 /// Returns a [Future] that asynchronously completes to `null`.
-Future get async => new Future.delayed(0, () => null);
+Future get async => new Future.delayed(const Duration(milliseconds: 0),
+                                       () => null);