Put babel back.

rollup.config.js

@@ -1,6 +1,7 @@
 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';
@@ -36,6 +37,9 @@ const plugins = [
   }),
   typescript(),
   json(),
+  babel({
+    exclude: 'node_modules/**',
+  }),
   dev && serve(serveopts),
   !dev && terser(),
 ];