I'm not currently serializing the .stack property
That's OK for now, but please file a followup on that? We should figure out how we want to do this... I do wonder whether some of the other deserialization issues I think I ran into while reviewing might have been an issue there.
If there's a better way to test the Error prototype than what I've done here, I'd appreciate any input
That's really a question for a SpiderMonkey peer. I asked Jeff to review the patch too.
Given this, I have interpreted the following from the spec to include those fields:
Yes, we should include them.
What should be covered in jsapi tests vs "non262" vs wpt ?
That's a good question. My gut feeling is that if it needs the web platform to test in general (i.e. it can't be a pure tes262 test), it should likely go in wpt. But again, this is largely a question for a SpiderMonkey peer.