aspirational approach to differential bundling

rollup/client.config.js

@@ -2,11 +2,13 @@ import resolve from 'rollup-plugin-node-resolve';
 import replace from 'rollup-plugin-replace';
 import commonjs from 'rollup-plugin-commonjs';
 import svelte from 'rollup-plugin-svelte';
+import babel from 'rollup-plugin-babel';
 import { terser } from 'rollup-plugin-terser';
 import config from 'sapper/config/rollup.js';
 
 const mode = process.env.NODE_ENV;
 const dev = mode === 'development';
+const legacy = process.env.SAPPER_LEGACY_BUILD;
 
 export default {
 	input: config.client.input(),
@@ -23,6 +25,15 @@ export default {
 			hydratable: true
 		}),
 
+		legacy && babel({
+			extensions: ['.js', '.html'],
+			include: [
+				'app/**',
+				'routes/**'
+			],
+			runtimeHelpers: true
+		}),
+
 		!dev && terser({
 			module: true
 		})