code: public org.inferred.freebuilder.processor.property.Property build() {
      Preconditions.checkState(_unsetProperties.isEmpty(), "Not set: %s", _unsetProperties);
      return new Value(this);
    }
docs: Returns a newly-created {@link org.inferred.freebuilder.processor.property.Property} based on
  the contents of this {@code Builder}.
  
  @throws IllegalStateException if any field has not been set