optional-polyfill.js

var scripts = ['app.a700a9a3e91a84de5dc0.js']; // script for all users

var newBrowser = (
  'fetch' in window &&
  'Promise' in window &&
  'assign' in Object &&
  'keys' in Object
);

if (!newBrowser) {
  scripts.unshift('polyfills.a700a9a3e91a84de5dc0.js'); // script for the less fortunate
}

scripts.forEach(function(src) {
  var scriptEl = document.createElement('script');
  scriptEl.src = src;
  scriptEl.async = false;
  document.head.appendChild(scriptEl);
});