private static ImmutableList<Property> primitivePropertiesIn(
        ImmutableSet<Property> properties) {
      return properties.stream().filter(p -> p.getKind().isPrimitive()).collect(toImmutableList());
    }
Subclass for when there are no Kotlin default properties.
value/src/main/java/com/google/auto/value/processor/BuilderRequiredProperties.java