SwitchPoint invalidation not working over prototype chain

The following script:

    Object.prototype.a = 1;
    var x = {};
    var y = Object.create(x);

    function p() {
        print("y.a: " + y.a);
    }

    p();
    // Delete, then redefine property
    delete Object.prototype.a;
    Object.prototype.a = 10;
    p();

should print:

y.a: 1
y.a: 10

but currently prints:

y.a: 1
y.a: undefined