Optimize Map.prototype.getOrInsertComputed #1157
Description
It currently searches the map twice (both before and after calling the user-provided JS function) because the user's function can change the map.
That's uncommon though, and can be avoided by adding a change counter to JSMapState. If the counter didn't change, the user's function didn't change the map, and we don't have to search again.
It means the set and delete operations have to do more work though, so I'm not fully convinced it's a net win. I'm opening this as a reminder to myself so I don't forget.