Phan can already handle PHP native assertions, so if you have this code
$x = getUnknownType(); assert( $x instanceof MyClass );
phan will know that $x is instance of MyClass.
However assert() cannot be used in MediaWiki, and we have the Assert library for that need. Thus, it'd be great to create a plugin to tell phan how our Assert::* functions behave. At least some of them are easy, e.g. Assert::parameterType.