client.js

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

loadPolyfills().then(mountApp);

loadPolyfills.js

import 'core-js/es6/promise';

export default function loadPolyfills() {
  const fillFetch = () => new Promise((resolve) => {
    if ('fetch' in window) return resolve();

    require.ensure([], () => {
      require('whatwg-fetch');

      resolve();
    }, 'fetch');
  });

  const fillIntl = () => new Promise((resolve) => {
    if ('Intl' in window) return resolve();

    require.ensure([], () => {
      require('intl');
      require('intl/locale-data/jsonp/en.js');

      resolve();
    }, 'Intl');
  });

  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([
    fillFetch(),
    fillIntl(),
    fillCoreJs()
  ]);
}