anchor.diff:    /// 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;
  +    final api = oauth2_v2.Oauth2Api(client);
       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);
  +      final info = await api.tokeninfo(accessToken: accessTokenString);
  +      if (info != null &&
  +          info.audience == _pubAudience &&
  +          info.expiresIn != null &&
  +          info.expiresIn > 0 &&
  +          info.verifiedEmail != null &&
  +          info.verifiedEmail &&
  +          info.email != null &&
  +          info.email != '') {
  +        return info.email;
  +      }
  +      if (info != null && info.audience != _pubAudience) {
  +        _logger.warning('OAuth2 access attempted with invalid audience, '
  +            'for email: "${info.email}", audience: "${info.audience}"');
  +      }
  +    } on oauth2_v2.ApiRequestError catch (e) {
  +      _logger.log(Level.INFO, 'Access denied for OAuth2 bearer token.', e);
       } catch (e, st) {
  -      _logger.warning('OAth2 bearer token lookup failed.', e, st);
  -    } finally {
  -      authClient.close();
  +      _logger.warning('OAuth2 bearer token lookup failed.', e, st);
       }
       return null;
     }
  
positive.code: 
  import 'package:gcloud/service_scope.dart' as ss;
  import 'package:googleapis/oauth2/v2.dart' as oauth2_v2;
  import 'package:googleapis_auth/auth_io.dart';
  import 'package:http/http.dart' as http;
  
  final _logger = new Logger('frontend.oauth2');
  
negative.code: 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;
  