anchor.diff:    @override
     Stream<String> validateEntity(FileSystemEntity entity) {
       assert(entity != null);
  -    return _streamFromIterableFuture(_getValidation(entity));
  +    return _oneOrNoneOnNull(_getValidation(entity));
     }
   
  -  Future<List<String>> _getValidation(FileSystemEntity entity) {
  +  Future<String> _getValidation(FileSystemEntity entity) {
       assert(entity != null);
   
       final entType = _getType(entity);
  
positive.code:     var future = fileSha1Hex(entity)
          .then((String sha1) {
            if(sha1 == targetSha) {
              return [];
            } else {
              return ['content does not match: $entity'];
            }
          });
  
      return _streamFromIterableFuture(future);
    }
  
    static Stream<String> validateDirectoryFromMap(FileSystemEntity entity,
  
negative.code:       return _exists(entity)
            .then((bool exists) {
              if(exists) {
                return null;
              }
              return "$entity does not exist on disk";
            });
      }
  
      return new Future.value(
          "Expected $entity to be $entityType, but it is $entType");
    }
  
    static Future<bool> _exists(FileSystemEntity entity) {
  