function assertEqual(actual, expected) {
    if (actual != expected)
        throw "Failed: actual: " + actual + ", expected: " + expected;
}

var a0 = [,,,,,,,,,,,,,];

Array.prototype.constructor = {
    [Symbol.species]: function() {
        return a0;
    }
}

var a1 = [1,2,3,4];
var a2 = a1.concat(a0);

assertEqual(a0, a2);
assertEqual(a0, "1,2,3,4,1,2,3,4,1,2,3,4,1,2,3,4,1");