I'm +1 on Nick's recommendation.

@Antoine
> Ideally, we would want to test both versions, so that any oddity in
> the freezing mechanism gets exercised and diagnosed properly.

+1

Does this mean that the whole test suite should be run under both (whenever _bootstrap.py is modified)?  Would that warrant a new flag for the test suite or even an automated check?

That's what I was getting at with this:

> 1. python starts up normally.
> 2. we clear out all the entire import state except for builtins.
> 3. we stick importlib._bootstrap in place.
> 4. we set builtins.__import__ to importlib.__import__.
> 5. we re-populate sys.modules by reloading all the modules that
> were in there before (?).
> 6. we run the test suite against this new import state.