code: public Datatype.Visibility getValueTypeVisibility() {
      Preconditions.checkState(
          !_unsetProperties.contains(Property.VALUE_TYPE_VISIBILITY), "valueTypeVisibility not set");
      return valueTypeVisibility;
    }
docs: Returns the value that will be returned by {@link Datatype#getValueTypeVisibility()}.
  
  @throws IllegalStateException if the field has not been set