This commit includes two changes:
1. It changes the unreliable dynamic way of loading IE polyfills to use
`<script nomodule>` instead - for IE it's equivalent to a regular script tag
while modern browsers will ignore it.
2. It removes other polyfills for browsers not supporting `Object.assign` as
this API is supported by Chrome 45+, Firefox 34+ and Safari 9+ i.e. it's been
supported for some time.
Note that as of June 2018 Googlebot uses Chrome 41 to render sites to be
indexed. Chrome 41 doesn't support `Object.assign` but it also doesn't support
ES6 modules so it'll load polyfills meant for IE - which it should do anyway
as it doesn't support most of ES6.
Fixes#23647
PR Close#24317
8be6892777