I forgot about that when writing my comment but yes, V8 made that change along with "shadow classes" or whatever the object specialization stuff was called. Heady days for JS performance.
In retrospect if something was going to be standardized I'd have preferred it to be the older behavior which was simpler to explain, but so it goes.
SpiderMonkey actually had inline caches before V8 shipped if I'm not mistaken; the motivating factor for the behaviour change in V8 was just to improve the cache-hit ratio.
In retrospect if something was going to be standardized I'd have preferred it to be the older behavior which was simpler to explain, but so it goes.