Add terser back.

rollup.config.js

@@ -2,6 +2,7 @@ import typescript from 'rollup-plugin-typescript2';
 import commonjs from '@rollup/plugin-commonjs';
 import nodeResolve from '@rollup/plugin-node-resolve';
 import babel from '@rollup/plugin-babel';
+import { terser } from 'rollup-plugin-terser';
 import serve from 'rollup-plugin-serve';
 import json from '@rollup/plugin-json';
 import styles from 'rollup-plugin-styles';