Add required polyfills so react-dates works in Internet Explorer 11 (https://github.com/woocommerce/woocommerce-admin/pull/356)

react-dates developers recommend using airbnb-browser-shims, but doing some
testing I found loading the required polyfills individually instead of the
entire Airbnb Browser Shims package produces smaller builds.

plugins/woocommerce-admin/client/components/calendar/index.js

@@ -4,6 +4,8 @@
  */
 import { Component } from '@wordpress/element';
 import moment from 'moment';
+import 'core-js/fn/object/assign';
+import 'core-js/fn/array/from';
 import {
 	DayPickerRangeController,
 	isInclusivelyAfterDay,