feat(elements): add polyfill for elements es5 shim (#22413)

PR Close #22413

aio/scripts/_payload-limits.json

@@ -4,7 +4,7 @@
       "uncompressed": {
         "inline": 2062,
         "main": 467103,
-        "polyfills": 54253,
+        "polyfills": 55349,
         "embedded": 71711,
         "prettify": 14888
       }

aio/src/polyfills.ts

@@ -30,9 +30,12 @@
 /** HACK: force import of environment.ts/environment.prod.ts to load env specific polyfills */
 import './environments/environment';
 
-/** window.customElements */
+/** Add support for window.customElements */
 import '@webcomponents/custom-elements/custom-elements.min';
 
+/** Required for custom elements for apps building to es5. */
+import '@webcomponents/custom-elements/src/native-shim';
+
 /** ALL Firefox browsers require the following to support `@angular/animation`. **/
 // import 'web-animations-js';  // Run `npm install --save web-animations-js`.