anchor.diff:      String object,
       Duration lifetime, {
       String? successRedirectUrl,
  -    String predefinedAcl = 'project-private',
       int maxUploadSize = maxUploadSize,
     }) async {
       final now = clock.now().toUtc();
  
positive.code: 
      final fields = {
        'key': key,
        'acl': predefinedAcl,
        'Expires': expirationString,
        'GoogleAccessId': result.googleAccessId,
        'policy': policyString,
  
negative.code:     final key = '$bucket/$object';
      final conditions = [
        {'key': key},
        {'expires': expirationString},
        if (successRedirectUrl != null)
          {'success_action_redirect': successRedirectUrl},
  