Change Future.delayed to take a Duration.

Review URL: https://codereview.chromium.org//12224081

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@18471 260f80e4-7a28-3924-810f-c04153c831b5
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index bcff589..83fe2b7 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -113,4 +113,5 @@
 }
 
 /// 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);