anchor.diff:  void registerOAuth2Service(OAuth2Service service) =>
       ss.register(#_oauth2_service, service);
   
  +/// The pub client's OAuth2 identifier.
  +final _pubAudience = '818368855108-8grd2eg9tj9f38os6f1urbcvsq399u8n.apps.'
  +    'googleusercontent.com';
  +
   /// A service used for looking up email addresses using an OAuth2 access token.
   class OAuth2Service {
     final http.Client client;
  
positive.code:   /// Looks up the email address by using the [accessTokenString] which
    /// contains an access token.
    Future<String> lookup(String accessTokenString) async {
      final future = new DateTime.utc(4242);
      final accessToken = new AccessToken('Bearer', accessTokenString, future);
      final credentials = new AccessCredentials(accessToken, null, []);
      final authClient = authenticatedClient(client, credentials);
  
      oauth2_v2.Userinfoplus info;
      try {
        final api = new oauth2_v2.Oauth2Api(authClient);
        info = await api.userinfo.get();
        return info.email;
      } on AccessDeniedException catch (e, st) {
        _logger.log(Level.INFO, 'Access denied for OAuth2 bearer token.', e, st);
      } catch (e, st) {
        _logger.warning('OAth2 bearer token lookup failed.', e, st);
      } finally {
        authClient.close();
      }
      return null;
    }
  
negative.code: 
  import 'package:gcloud/service_scope.dart' as ss;
  import 'package:googleapis/oauth2/v2.dart' as oauth2_v2;
  import 'package:http/http.dart' as http;
  
  final _logger = new Logger('frontend.oauth2');
  