\documentclass[beamer]{standalone}
\usepackage{tikz}
\usetikzlibrary{external}
\tikzexternalize % activate! 
\begin{document}
\begin{standaloneframe}

% If overlays do not work, use \only<1-n>{...} where n is the max overlay
% \only<1-1000>{
  \begin{tikzpicture}[]
    \draw[line width=2] (0,0) -- (0,1);
    \draw[line width=2] (-1,2) -- (0,1) -- (1,3);
    \draw[fill=black] (-1, 2) circle (0.15);
    \draw[fill=black] (1,3) circle (0.15);
    \draw[dashed] (-1.5, 1) -- (1.5,1);
    \draw[dashed] (-1.5, 2) -- (1.5,2);
    \draw[dashed] (-1.5, 3) -- (1.5,3);
  \end{tikzpicture}
% }
\end{standaloneframe}
\end{document}