code: static ImmutableList<VariableElement> getFieldsToParcel(
        TypeElement element, OptionsDescriptor options) {
      Optional<AnnotationMirror> paperParcelMirror =
          MoreElements.getAnnotationMirror(element, PaperParcel.class);
      if (paperParcelMirror.isPresent()) {
        ImmutableList.Builder<VariableElement> fields = ImmutableList.builder();
        getFieldsToParcelImpl(element, options, fields, new HashSet<Name>());
        return fields.build();
      } else {
        throw new IllegalArgumentException("element must be annotated with @PaperParcel");
      }
    }
docs: Returns all non-excluded fields on a {@link PaperParcel} annotated {@link TypeElement}.