client.js

import loadPolyfills from './load-polyfills';
import mountApp from './app'; // the entry point for the rest of my app

loadPolyfills().then(mountApp);

load-polyfills.js

import 'core-js/es6/promise';

export default function loadPolyfills() {
  const fillCoreJs = () => new Promise((resolve) => {
    if (
      'startsWith' in String.prototype &&
      'endsWith' in String.prototype &&
      'includes' in Array.prototype &&
      'assign' in Object &&
      'keys' in Object
    ) return resolve();

    require.ensure([], () => {
      require('core-js');

      resolve();
    }, 'core-js');
  });

  return Promise.all([
    fillCoreJs()
  ]);
}

Removed Fetch and i18n conditional polyfills as they are often decided by developers or are a single breakpoint that could be solved by reverting to the original code.