SSAS change in FMS 4 = VM update?

The following code behaves differently in FMS 3.5 vs. FMS 4:

    var loops;
    loops = 0;
    var obj = {
        r1: 'v1',
        r2: 'v2'
    };
    for(var iter in obj)
    {
        delete obj['r1'];
        delete obj['r2'];
        loops++;
    }
    Tester.assert(loops == 2, 'Deleting items in an object while iterating does not affect the iteration.');

FMS 3.5 ends up with 2 loops, FMS 4 ends up with 1 (The correct behavior).  Was the JavaScript VM updated?