Authoritative answer on which boots first – Plugins or Themes?

I can’t seem to find anywhere in the WordPress documentation an authoritative answer on the question on whether a WordPress plugin boots first or whether a WordPress theme boots first.

I need to know the answer because an important development decision I am working on depends on it – as it is related to themes that depend on the presence of certain plug-ins.

And by “authoritative answer”, I mean — is the answer as-it-is just an accident of how WordPress is presently designed – subject to being all up-in-the-wind next version of WordPress — or is it a standard aspect of how WordPress is supposed to work?

See this famous answer by the equally famous Rarst. Here he charts out the load process of WordPress which hasn’t and isn’t expected to change any time soon. The process pretty much goes:

1. WordPress Core
2. Must-Use Plugins ( mu-plugins directory )
3. Plugins
4. Themes ( Child before Parent )