Later versions of Twig add a $throw parameter to Twig loaders' findTemplate() calls. Our ThemeRegistryLoader doesn't have this so it needs to be added.
Why this should be an RC target
This is not disruptive, we should do it before 8.0.0 so that we are ready to upgrade to Twig 2.x at a later time.
Per @effulgentsia in:
I discussed this with the other committers, and yes to removing Drupal core usages of @deprecated Twig 1.x APIs being "rc target" material.
User interface changes
Should be none.
Data model changes