Today, it's not.
V8 bindings use a magic property to indicate overriding of .all property with additional logic in named property getter and setter.
JSC bindings use direct set/put pair of methods.
We should write a layout test to check all corner cases and make both implementations match.
This would be nice to do for cleanup reasons, but I'm going to remove it from blocking https://bugs.webkit.org/show_bug.cgi?id=32630, as it's not really adding a maintenance burden.
Closing some V8-related work items.