So siehts ganz gut aus !

node_modules/acorn/rollup/config.main.js

@@ -0,0 +1,11 @@
+import buble from 'rollup-plugin-buble';
+
+export default {
+	entry: 'src/index.js',
+	moduleName: 'acorn',
+	plugins: [ buble() ],
+	targets: [
+		{ dest: 'dist/acorn.js', format: 'umd' },
+		{ dest: 'dist/acorn.es.js', format: 'es' }
+	]
+};