<!DOCTYPE html>
<x-b id="b1"></x-b>
<script>
(function() {
    var proto = Object.create(HTMLElement.prototype);
    proto.createdCallback = function () {
        window.upgradeIds.push(this.id);
    };

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

test(function () {
    assert_array_equals(window.upgradeIds, ['a1', 'a2', 'a3', 'b0', 'b1'], 'Elements should got upgrade from the import.');
}, 'upgrade in an import (in nested import)');
</script>
<x-b id="b2"></x-b>