Calling ScriptObject.setProto() is quite expensive. One expensive part is invalidating the object's property map, which involves a WeakHashMap lookup in PropertyMap.checkProtoHistory().

Invaliding the property map is necessary if the property map may be in use with other prototypes. However, many built-in objects including arrays (in fact all built-ins except for Object) are always constructed with the same combination of prototype and property map.

In this case we can significantly improve object creation overhead by providing a way to directly set both prototype and property map.