Previously scripts were failing to find attributes that they assigned to
window and then tried to use from the global scope. Eg
window.newthing = function() {...};
newthing(...); // newthing is not defined error
This wasn't the case for things that already existed in the global scope
and were just being overwritten.
This change just overrides the `Proxy.has()` function which seems to fix
it. Probably the `while` implementation was failing to pick up new
attributes because of the lack.
I also tweaked some comments and variable names and const-ness to be a
little more production ready.
23bfe6daa2