To be fair, the old async rewriter could also basically just not handle this because it didn’t allow this in the mapping function, where you’d have to use it in order to get sensible results. This would ideally work by making Function.prototype.toString() return the right result here. If that’s infeasible, it should also work by always defining the helper functions (isSyntheticPromise etc.) instead of re-using ones from existing outer scopes.