static/ext/src/lang/Array.js
function (array, begin, end) {
                // After tested for IE 6, the one below is of the best performance
                // see http://jsperf.com/slice-fix
                if (typeof begin === 'undefined') {
                    return slice.call(array);
                }
                if (typeof end === 'undefined') {
                    return slice.call(array, begin);
                }
                return slice.call(array, begin, end);
            }
function (array, begin, end) {
                // After tested for IE 6, the one below is of the best performance
                // see http://jsperf.com/slice-fix
                if (typeof begin === 'undefined') {
                    return slice.call(array);
                }
                if (typeof end === 'undefined') {
                    return slice.call(array, begin);
                }
                return slice.call(array, begin, end);
            }
at least IE6 uses arguments.length for variadic signature