function render() {
      var checked = radioGroupController.getViewValue() == attr.value;

      if (checked === lastChecked) return;

      if (element[0] && element[0].parentNode &&
          element[0].parentNode.nodeName.toLowerCase() !== 'md-radio-group') {
        // If the radioButton is inside a div, then add class so highlighting will work.
        element.parent().toggleClass(CHECKED_CSS, checked);
      }

      if (checked) {
        radioGroupController.setActiveDescendant(element.attr('id'));
      }

      lastChecked = checked;

      element
        .attr('aria-checked', checked)
        .toggleClass(CHECKED_CSS, checked);
    }
Add or remove the `.md-checked` class from the RadioButton (and conditionally its parent).
Update the `aria-activedescendant` attribute.