code: @Override
      public byte[] retrieveBytes(File impFi) {
          if ( resourceLocator != null ) {
              // does only return a result if a transpiler is registered
              byte[] bytes = resourceLocator.retrieveBytes(impFi);
              if ( bytes != null )
                  return bytes;
          }
          return HtmlImportShim.ResourceLocator.super.retrieveBytes(impFi);
      }
docs: this part of locator interface is only implemented if another
  resourcelocator is set.
  
  @param impFi
  @return