Fix OAuth2 tests, and disable them on non-dart:io platforms.

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';
 
+import 'credentials.dart';
+
 /// 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.";
 }