It's not possible in JS to directly create a function object with a
custom prototype. We previously got around this by directly altering the
prototype by setting the __proto__ property, but SpiderMonkey now
conspicuously warns that this will make your code slow.

It would be possible to do this with ES6 Proxy objects, although
SpiderMonkey 31 doesn't support the particular getPrototypeOf() proxy
trap that we would need in order to implement this correctly --- at
least not in JS. Therefore we implement the proxy in C++.

We add a debug topic for proxies and a memory counter.

All in all, the proxy is probably still slower than a function object
with a real prototype would be, but hopefully faster than direct
alteration of the prototype. At the very least we can avoid printing a
big warning every time our class framework is used.

https://bugzilla.gnome.org/show_bug.cgi?id=751252