rollup: specify generatedCode: 'es2015' (#35070)

This will make use of `const` and modern features (which we are already using in our code) in the generated rollup code.

js/tests/karma.conf.js

@@ -98,7 +98,8 @@ const conf = {
     output: {
       format: 'iife',
       name: 'bootstrapTest',
-      sourcemap: 'inline'
+      sourcemap: 'inline',
+      generatedCode: 'es2015'
     }
   }
 }