<!DOCTYPE html>
<script>
(function() {
    var proto = Object.create(HTMLElement.prototype);
    proto.createdCallback = function () {
        window.upgradeIds.push(this.id);
    };

    var ctor = document.registerElement('x-a', {prototype: proto});
})();

test(function () {
    assert_array_equals(window.upgradeIds, ['a1'], "This import shouldn't upgrade elements built by overrunning parser.");
}, 'upgrade in an import');
</script>