Make oauth2 warning-clean.

pkg/oauth2/lib/src/credentials.dart

Index: pkg/oauth2/lib/src/credentials.dart
diff --git a/pkg/oauth2/lib/src/credentials.dart b/pkg/oauth2/lib/src/credentials.dart
index 26112c63a50a08f0ca75147bd6532af9ae632d4e..acc5690256e733c1e2584f6fb4d42b9b4b6879c0 100644
--- a/pkg/oauth2/lib/src/credentials.dart
+++ b/pkg/oauth2/lib/src/credentials.dart
@@ -50,7 +50,8 @@ class Credentials {
   /// 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;