Previously, the `elements` docs example only worked in browsers that
natively supported Custom Elements and ES2015 modules. Furthermore, it
didn't work on StackBlitz, because StackBlitz ignores the specified
`target` in `tsconfig.json` and uses the UMD bundles (i.e. ES5 code)
even on browsers that do support ES2015.
(NOTE: In the past, this was not a problem, because we explicitly did
not provide a StackBlitz example. This has changed in #36067.)
This commit ensures the example works on all browsers and also on
StackBlitz by providing the necessary Custom Elements polyfills.
Fixes#36532
PR Close#36536
2e4244d521