I think this is just fragility, not cheating. I don't know JS super well, but in some languages you might see some rules associated with certain operations and preserving over/under flow exceptions and such.

In any case I think a few things happened here:

1) For whatever reason the "true" statement caused the compiler to think there was a side-effect potential. I suspect the compiler simply didn't know what to do with it, and they hadn't handled 'true;' or 'false;' as standalone statements in their optimizer. I bet if you put 'true;' in the middle of that loop it will break the DCE.

2) The probably don't do liveness analysis. So they can see that a block doesn't change global state, but don't look to see if the proceeding blocks use any of the variables. So if there is any code after a block they assume that they can't DCE that block.

3) '*' and '%' causing problems may be very specific to those operations, and I'm guessing '/' too.

All in all I'd say it is a target incomplete implementation, but not cheating. Based on what I've seen thus far.