Redesign toggle is broken in Storybook

Created by: valerybugakov

Steps to reproduce:

1. yarn storybook on the main branch or open production build http://storybook.sgdev.org/.
2. try toggle the redesign theme using the toolbar icon.
3. it's possible to enable the redesign theme. But then toggle does nothing.

Expected behavior:

Redesign toggle enables/disables redesign theme.

Notes:

The issue is caused by Babel config and became apparent after merging #20228.