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