Packaging issue with @jupyter-widgets/base and backbone when loaded as an esm module

[paddymul]

Description

I am trying to package my widget as a standalone npm module for use in codepen. When packaged with jsdelivr in esm mode, I get the following error loading the JS.

Uncaught TypeError: class heritage n.View is not an object or null
    <anonymous> nativeview.js:53

Reproduce

1. Go to https://codepen.io/paddymul/pen/qBerprv
2. https://codepen.io/paddymul/pen/qBerprv
3. Open the JS console
4. Look at the type error from nativeview

Expected behavior

I would expect the

Context

This is built from a branch of my code
paddymul/buckaroo#298

I don't think the rest of the fields are relevant since this is a JS bug. But here are the relevant JS config files

Webpack from the branch
https://github.com/paddymul/buckaroo/blob/chore/js-0.7.5-test9-reproduce/webpack.config.js
package.json from the branch
https://github.com/paddymul/buckaroo/blob/chore/js-0.7.5-test9-reproduce/package.json

Package built with yarn run build