As I said:

The only case that arr.length could change, and hence the if statement taken, is if the if statement had already been taken - i.e. arr.length had already changed. This seems safe to optimize.

It doesn't matter what H.throwConcurrentModificationError is - it can only be called iff it has already been called. Which means that as soon as you're through the first loop iteration, you know that arr.length hasn't changed that loop, and it won't change any other iterations through the loop.

And as for JS - there are plenty of slide decks that work perfectly fine without, or at the very least degrade gracefully. So that's not a good excuse.