Don't try to parse expression closure bodies as comma expressions.

This is one of the two `expression(0)` calls that I forgot to update. Fixes #1089.

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The code for `stmt("yield", ...)` needs to be changed to `expression(5)` too, but the full fix is a little bit more involved than that, so I'm putting it off for now (If I try hard, I could even pretend I purposely didn't update it before).

- For starters, `yield` needs to be an expression, i.e. `prefix`, not `stmt`.
- Second, `yield` is parsed differently by Mozilla vs the official es6 spec, so things could get hairy if you expect to handle both cases correctly. I believe the difference is just that Mozilla requires parens in cases where the es6 spec doesn't (because those cases are handled by operator precedence), but I haven't looked at the issue in awhile.