Modern set operations arrive and the language reaches full conformance

Modern set operations arrive, and the language layer reaches full standard conformance. The Set type, used to hold a collection of unique values, now offers the newest standard operations for combining and comparing sets directly: joining two sets, finding what they share, what one has that the other lacks, and checking whether one contains or overlaps another. These work with any set-like value, not only another set, and reject anything that is not. With this in place the from-scratch JavaScript engine now matches the standard across the language surface it targets. Code that was already correct is unchanged. The JavaScript language aspect moves 99 to 100; the overall platform score stays at 70.