Fixes in `admin-bar.js`:

- Silence errors when a node doesn't exist similarly to jQuery.
- Add "feature testing" and fallbacks for old browsers as this may run on the front-end.
- Improve inline docs.

Props dinhtungdu, azaozz.
Fixes #47069.
Built from https://develop.svn.wordpress.org/trunk@46883


git-svn-id: https://core.svn.wordpress.org/trunk@46683 1a063a9b-81f0-0310-95a4-ce76da25c4cd