\documentclass{article}

\begin{document}

\begin{DIFnomarkup}
{\small
\catcode`!=\active
\begin{Verbatim}[frame=single,numbers=left,defineactive=\def!{\bfseries}]
 {
   // ... preamble ...
   // ... postamble ...
 }
\end{Verbatim}
}
\end{DIFnomarkup}
\end{document}
