function sortObjectsByKey(arr, key, opt_compareFn) {
  sortByKey(arr, function(obj) {
    return obj[key];
  }, opt_compareFn);
}
Sorts an array of objects by the specified object key and compare
function. If no compare function is provided, the key values are
compared in ascending order using <code>defaultCompare</code>.
This won't work for keys that get renamed by the compiler. So use
{'foo': 1, 'bar': 2} rather than {foo: 1, bar: 2}.
@param {Array<Object>} arr An array of objects to sort.
@param {string} key The object key to sort by.
@param {Function=} opt_compareFn The function to use to compare key
    values.