This fixes the following error, which occurs when running $ npm start after $ npm run build:
ReferenceError: regeneratorRuntime is not defined
at ...
at Module._compile (module.js:641:30)
at Object.Module._extensions..js (module.js:652:10)
at Module.load (module.js:560:32)
at tryModuleLoad (module.js:503:12)
at Function.Module._load (module.js:495:3)
at Module.require (module.js:585:17)
at require (internal/module.js:11:18)
I had the same problem as you.
I solved it using your solution but you should runnpm run babel-polyfill --save-dev, to find this module in package.json.
This fixes the following error, which occurs when running
$ npm start
after$ npm run build
: