code: function setAriaControls (tab) {
      if (tab.hasContent) {
        var nodes = $element[0].querySelectorAll('[md-tab-id="' + tab.id + '"]');
        angular.element(nodes).attr('aria-controls', ctrl.tabContentPrefix + tab.id);
      }
    }
docs: Sets the `aria-controls` attribute to the elements that correspond to the passed-in tab.
  @param tab