code: function lowercase(arr) {
    var ret = new Array(arr.length);
  
    for (var i = 0; i < arr.length; ++i) {
      ret[i] = String(arr[i]).toLowerCase();
    }
  
    return ret;
  }
docs: Lowercase the given `arr`.
  
  @api private
  @param {Array} arr
  @return {Array}