We treat all NativeFunctions as strict mode and thus window function
which were called in a global context (i.e. `setTimeout(f, 0)`) got a
null this_value. But we really need to treat all functions not defined
by the ECMAScript specification as non-strict. In most cases this won't
matter however since Window is also the global_object we have an extra
bit of logic.
To fix this more correctly we would need to track the strictness of
NativeFunctions.
e22539bdd9