Minor fix: Please change PLUGINDIR to WP_PLUGIN_DIR

When using a custom path to WP_CONTENT (such as what is used in https://roots.io/bedrock/) and using mu-plugins, the code you have here: https://plugins.trac.wordpress.org/browser/wordpress-mu-domain-mapping/trunk/domain_mapping.php#L679 breaks.

Specifically, because you are using a deprecated constant PLUGINDIR. The proper constant to use is WP_PLUGIN_DIR. See: https://github.com/WordPress/WordPress/blob/master/wp-includes/default-constants.php#L145-L188

This causes something like http://example.com/wp/mu-plugins/pluginname/file.js to NOT match with PLUGINDIR, but strip off the last character of the file (“s”), and then return a new plugin URL with “s” appended to the site URL:

http://example.coms/

Simply replacing PLUGINDIR with WP_PLUGIN_DIR would correct the problem. Thanks! In the meantime, I guess we’ll be defining PLUGINDIR locally.