Flow and MobileFrontend want to share code that is not quite ready for MediaWiki core (core/mantle - get it?). One big example is sharing of delivering of client side templates via ResourceLoader.
MobileFrontend has been doing this for some time, and the new extension Mantle simply shuffles code from there into a standalone repository so that Flow can use it without being dependent on MobileFrontend.
Sadly this is not quite ready for core - as there has been no standardisation on a default template library.
Once deployed, MobileFrontend can add a hard dependency to Mantle by merging the following patch:
Read more about Mantle here: https://www.mediawiki.org/wiki/Extension:Mantle
Note: The plan is also to move Nearby functionality into the GeoData extension and add a dependency on GeoData to Mantle.