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

pkg/oauth2/test/authorization_code_grant_test.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 authorization_code_grant_test;
 
 import 'dart:io';
 import 'dart:json';
 import 'dart:uri';
 
 import '../../unittest/lib/unittest.dart';
 import '../../http/lib/http.dart' as http;
 import '../../http/lib/testing.dart';
 import '../lib/oauth2.dart' as oauth2;
 import 'utils.dart';
 
 final redirectUrl = new Uri.fromString('http://example.com/redirect');
 
 ExpectClient client;
 
-AuthorizationCodeGrant grant;
+oauth2.AuthorizationCodeGrant grant;
 
 void createGrant() {
   client = new ExpectClient();
   grant = new oauth2.AuthorizationCodeGrant(
       'identifier',
       'secret',
       new Uri.fromString('https://example.com/authorization'),
       new Uri.fromString('https://example.com/token'),
       httpClient: client);
 }
@@ skipping 155 matching lines @@
       });
 
       expect(grant.handleAuthorizationCode('auth code'),
           completion(predicate((client) {
             expect(client.credentials.accessToken, equals('access token'));
             return true;
           })));
     });
   });
 }