This project exposes a flaw with Chrome DevTools Node.js sourcemap support - source files are not displayed in the source file list.
Verification of existing index.js sourcemap -
Open index.html in chrome and look at the console output and source list. The script will output '4' but more importantly you'll see index.es6.js in the source list.
Chrome DevTools sourcemap failure -
Run npm start in the console and open the resulting url. Hit Resume on the debugger. The script will run to completion. Take a look at the source file list - index.es6.js is not present.
You can rebuild index.js/index.js.map with npm install && npm run build although I've checked in the results.