fix(docs-infra): use script nomodule to load IE polyfills, skip other polyfills (#24317)

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

aio/src/index.html

@@ -82,31 +82,7 @@
     };
   </script>
 
-  <script>
+  <script nomodule src="generated/ie-polyfills.min.js"></script>
-    if (window.document.documentMode) {
-      // polyfill IE11 in a blocking way
-      var s = document.createElement('script');
-      s.src = 'generated/ie-polyfills.min.js';
-      document.head.appendChild(s);
-    } else if (!Object.assign) {
-      // polyfill other non-evergreen browsers in a blocking way
-      var polyfillUrl = "https://cdn.polyfill.io/v2/polyfill.min.js?features=default,Array.prototype.find&flags=gated&unknown=polyfill";
-
-      // send a blocking XHR to fetch the polyfill
-      // then append it to the document so that its eval-ed synchronously
-      // this is required because the method used for IE is not reliable with other non-evergreen browsers
-      var xhr = new XMLHttpRequest();
-      xhr.addEventListener("load", function() {
-        var s = document.createElement('script');
-        s.type = 'text/javascript';
-        var code = this.responseText;
-        s.appendChild(document.createTextNode(code));
-        document.head.appendChild(s);
-      });
-      xhr.open("GET", polyfillUrl, false);
-      xhr.send();
-    }
-  </script>
 
   <script>
     //load CE polyfill