function arrayConcatSlice2(a1, a2, a3, start) {
    var result = a1.concat(a2);
    var l1 = result.length;
    var length = a3.length - start;
    result.length = l1 + length;
    for (var i = 0; i < length; ++i) {
        result[l1 + i] = a3[start + i];
    }
    return result;
}
Create a new array consisting of a1 + a2 + a subset of a3. Avoids allocating an extra array by calling `slice` on a3.