Add a package for authenticating via OAuth2.

pkg/oauth2/lib/src/expiration_exception.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 expiration_exception;
+
+import 'dart:io';
+
+/// An exception raised when attempting to use expired OAuth2 credentials.
+class ExpirationException implements Exception {
+  /// The expired credentials.
+  final Credentials credentials;
+
+  /// Creates an ExpirationException.
+  ExpirationException(this.credentials);
+
+  /// Provides a string description of the ExpirationException.
+  String toString() =>
+    "OAuth2 credentials have expired and can't be refreshed.";
+}