function updateInkBarClassName () {
    var elements = getElements();
    var newIndex = ctrl.selectedIndex,
        oldIndex = ctrl.lastSelectedIndex,
        ink      = angular.element(elements.inkBar);
    if (!angular.isNumber(oldIndex)) return;
    ink
        .toggleClass('md-left', newIndex < oldIndex)
        .toggleClass('md-right', newIndex > oldIndex);
  }
Adds left/right classes so that the ink bar will animate properly.