code: private List<EntityType> getNewSources(MappingTarget target) {
      return dataService
          .getEntityTypeIds()
          .filter(name -> isValidSource(target, name))
          .map(dataService::getEntityType)
          .collect(toList());
    }
docs: Lists the entities that may be added as new sources to this mapping project's selected target
  
  @param target the selected target