js-delete-prop was setting value to js-undefined instead of
removing the key, so 'key' in obj remained true and proto-chain
lookup didn't fall through. Switched to dict-delete!.
Now delete Boolean.prototype.toString; Boolean.prototype.toString()
walks up to Object.prototype.toString and returns "[object Boolean]".
built-ins/Boolean: 21/27 → 22/27. conformance.sh: 148/148.