function sort(arr, opt_compareFn) {
  // TODO(arv): Update type annotation since null is not accepted.
  arr.sort(opt_compareFn || defaultCompare);
}
Sorts the specified array into ascending order.  If no opt_compareFn is
specified, elements are compared using
<code>defaultCompare</code>, which compares the elements using
the built in < and > operators.  This will produce the expected behavior
for homogeneous arrays of String(s) and Number(s), unlike the native sort,
but will give unpredictable results for heterogeneous lists of strings and
numbers with different numbers of digits.

This sort is not guaranteed to be stable.

Runtime: Same as `Array.prototype.sort`

@param {Array<T>} arr The array to be sorted.
@param {?function(T,T):number=} opt_compareFn Optional comparison
    function by which the
    array is to be ordered. Should take 2 arguments to compare, and return a
    negative number, zero, or a positive number depending on whether the
    first argument is less than, equal to, or greater than the second.
@template T