Make oauth2 warning-clean.

Review URL: https://codereview.chromium.org//12379021

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@19262 260f80e4-7a28-3924-810f-c04153c831b5
diff --git a/lib/src/credentials.dart b/lib/src/credentials.dart
index 26112c6..acc5690 100644
--- a/lib/src/credentials.dart
+++ b/lib/src/credentials.dart
@@ -50,7 +50,8 @@
   /// credentials will expire shortly after this is called. However, since the
   /// client's expiration date is kept a few seconds earlier than the server's,
   /// there should be enough leeway to rely on this.
-  bool get isExpired => expiration != null && new DateTime.now() > expiration;
+  bool get isExpired => expiration != null &&
+      new DateTime.now().isAfter(expiration);
 
   /// Whether it's possible to refresh these credentials.
   bool get canRefresh => refreshToken != null && tokenEndpoint != null;