anchor.diff:    Future<dynamic> _put(
       Uri url, {
       Object? body,
  +    // TODO(chillers): Remove once b/239021831 has been fixed by the GoB side.
  +    String? branchName,
     }) async {
       final http.Client authClient = await authClientProvider(baseClient: httpClient, scopes: <String>[]);
       // GoB replicas may not have all the Flutter state, and can require several retries
  
positive.code:     String body = jsonEncode(<String, String>{
        'revision': revision,
      });
      final Map<String, dynamic> response = await _put(url, body: body) as Map<String, dynamic>;
      log.info(response);
      if (response['revision'] != revision) {
        throw const InternalServerError('Failed to create branch');
  
negative.code:       when: (http.BaseResponse response) => _responseIsAcceptable(response) == false,
        delay: (int attempt) => retryDelay ?? const Duration(seconds: 3) * attempt,
      );
      final http.Response response = await client.put(
        url,
        body: body,
        headers: <String, String>{
          // TODO(chillers): Remove once b/239021831 has been fixed by the GoB side.
          'X-Gerrit-Trace': 'bug-239021831-$branchName'
        },
      );
      if (_responseIsAcceptable(response) == false) {
        throw InternalServerError('Gerrit returned ${response.statusCode} which is not 200 or 202');
      }
  