Typing: Use nullthrows to fix a case where a value is never null

Summary: An example of how you might cover a case such as this, where we
can inspect the code and see that this member variable can never be null
here, and need to convince the type system of that.

Test Plan: Test runner.
Typing: Example of how to fix "callable"

Summary: I'm actually not sure what's up with the HHVM "callable" type
and need to go sync with those guys :) In any event, our type system
doesn't support it right now. This is one example of how you can fix
"callable", by being more specific in what your function type is. We
also have a couple magic functions for getting "function pointers" as
far as the type system is concerned (though they are just arrays at
runtime), let me know if you're interested -- unfortunately they don't
deal with LSB right now which I think is the form you need the most.

Test Plan: Test runner.