IsConstructor

[André Bargull]

> [*] Proxies are oddballs here. All Proxies have a [[Construct]] method so
> the IsConstructor will always return true which is really not what you
> want. If IsConstructor was changed to check for a .prototype instead
> proxies would behave more inline with ordinary objects.

[[Construct]] is only attached to a proxy if the target has a 
[[Construct]] internal method.