code: private IonDatagramLite load_helper(IonReader reader)
      throws IOException
      {
          IonDatagramLite datagram = new IonDatagramLite(_system, _catalog);
          IonWriter writer = _Private_IonWriterFactory.makeWriter(datagram);
          writer.writeValues(reader);
          return datagram;
      }
docs: This doesn't wrap IOException because some callers need to propagate it.
  
  @return a new datagram; not null.