Local arguments

Actually you should use `js.BinaryOp(js.BinaryOp.Int_-, argLen, argOffset)`, and similarly for the other operators on integers. This will make Desugaring emit the `| 0`, for which I now have sort of confirmation that they actually *improve* performance on recent browsers.
You can also use `js.BinaryOp(js.BinaryOp.<, counter, count)`, although that does not change anything, but for consistency.