polyfill.php
This polyfill does some (persnickity) deduplication, but it doesn't allow upgrades or shipping replacements in core.
Note: The polyfill.php is designed to be copied into extensions for interoperability. Consequently, this file is
not used 'live' by civicrm-core. However, the file does need a canonical home, and it's convenient to keep it
adjacent to the actual mixin files.