Remove redundant checker functions and use patterns more friendly to modules

[weswigham]

So there's a couple small things bundled in here:

1. getObjectFlags on checker was redundant. There's a ts.getObjectFlags that it was just a copy of. So it's gone now.
2. A shorthand reference to a namespace variable is kind of a pain to track and transform - fortunately we don't do it often, so it's easier to just change the one instance of it in our codebase (after checker.getObjectFlags is removed) to use a local variable.
3. (Module) Namespace members are not writable, though they are mutable - this means we can't actually assign to a namespace member, but we can can assign to the variable backing that namespace member and see it update. Two instances of this were changed. The assignments were in a separate file, so we had to introduce a helper function to perform the actual set.

Do note that I'm just opening PRs to fix/work around these things as they come up while I'm working on the module transition, and I figured it's much easier to review them in little bits like this, before the big switch over, assuming the new code still works both before and after the change.