Do you mean "only has floats"? That sucks. But okay, I see why Dylan wants to treat integer-valued floats as exact, even though technically that's wrong. Sigh.
P.S. But if we just always used the Scheme numeric tower, like real languages, then it'd all work perfectly!
of course I meant "floats", duh! 
Native big integers were actually added to JS recently, and they're now supported by the latest stable versions of every major browser. Just need to wait for users to update their Safari... BigInt | Can I use... Support tables for HTML5, CSS3, etc
I should probably look into replacing the Scheme arithmetic library with native big numbers... Also, maybe allow them to be used in Snap! without a library, now that they're a vanilla JS thing. 
Not unless it still supports exact rationals and complex numbers!
I haven't had much time to investigate, but my preliminary assessment is that biginteger.js could probably be eliminated, but a modified version of schemeNumber.js would need to stay (for the features you listed, fractions, etc.)
Maybe the first step should be an email to the schemeNumber person inquiring about plans to incorporate native bignums.
Nothing except the documentation has been updated in 8 years. I sort of doubt there are plans.
Ah, well, looks like you're about to hit the big time of free software!
Okay, at long last, I have fixed these problems, I believe. Next release...