Per our Super happy dev time meeting, we will be looking to consolidate code between Minerva and core wherever possible.
Back in the old days when Minerva/MobileFrontend was being built the watchstar in core was not seen as reusable enough to cover Minerva's multiple watchstars.
Given the looming desktop refresh (and the likelihood that minerva will go into maintenance) it seems favorable for Minerva to use the same watchstar as Vector.
= QA steps
 Click the watchstar when logged out - you should still see a CTA drawer.
 Login and click the watchstar. The watchstar should change. After refreshing the page, the watchstar should remain as it was.
 Login and do a search. Watch an article. Click on the search result and that article should be watched.
 Visit Special:EditWatchlist and unwatch an article - when the page is refreshed it should no longer be on the watchlist.