ES6 draft 03-17-15, section 22.1.3.3
function ArrayCopyWithin(target, start, end) {
  CHECK_OBJECT_COERCIBLE(this, "Array.prototype.copyWithin");

  var array = TO_OBJECT(this);
  var length = TO_LENGTH(array.length);

  return InnerArrayCopyWithin(target, start, end, array, length);
}