Javelin initBehavior is set to only source behaviors from the 'phabricator' library. This is because 'phabricator' is set as the source_name variable in CelerityStaticResourceResponse.
The source name identifies the location of the library map, so this prohibits 'load-libraries' type extensions to use behaviors. When Javelin::initBehavior is called adding a source_name argument would enable sourcing from other libraries outside of phabricator.
Perhaps Javelin.php could be changed so it passes $source_name and if $source_name is not set make it default to 'phabricator' there rather than in CelerityStaticResourceResponse.
This blocks the implementation of the Sprint extension planned for the Wikimedia instance.