code: function checkEntityTypes(em, entityTypes) {
      assertParam(entityTypes, "entityTypes").isString().isOptional().or().isNonEmptyArray().isString()
          .or().isInstanceOf(EntityType).or().isNonEmptyArray().isInstanceOf(EntityType).check();
      if (typeof entityTypes === "string") {
        entityTypes = em.metadataStore._getEntityType(entityTypes, false);
      } else if (Array.isArray(entityTypes) && typeof entityTypes[0] === "string") {
        entityTypes = entityTypes.map(function (etName) {
          return em.metadataStore._getEntityType(etName, false);
        });
      }
      return entityTypes;
    }
docs: takes in entityTypes as either strings or entityTypes or arrays of either and returns either an entityType or an array of entityTypes or throws an error