cross browser getPrototypeOf.

This breaks on custom prototypes who do not set `Prototype.prototype.constructor = Prototype` in oldIE.

Thankfully anyone using `util.inherits` will set [the constructor prototype properly](https://github.com/isaacs/inherits/blob/master/inherits_browser.js#L6)

Also all browsers set the constructor property on a prototype by default. it's only overwritten if you implement inheritance wrong.