Weird things are happening with internal slots in effectful generators

[NTillmann]

This serializer test fails.

(function () {
    let p = {};
    function f(c) {
        let o = {};
        if (c) {
            o.__proto__ = p;
            throw o;
        }
    }
    if (global.__optimize) __optimize(f);
    inspect = function() { try { f(true); } catch (e) { return e.$Prototype === p; } }
})();

The underlying issue is that a ModifiedPropertyEntry is created for $Prototype.

[NTillmann]

At the very least, or as the first step, this should be detected and an error should be logged.