code: function changeSelectedButton(parent, increment) {
      // Coerce all child radio buttons into an array, then wrap then in an iterator
      var buttons = $mdUtil.iterator(parent[0].querySelectorAll('md-radio-button'), true);
  
      if (buttons.count()) {
        var validate = function (button) {
          // If disabled, then NOT valid
          return !angular.element(button).attr("disabled");
        };
  
        var selected = parent[0].querySelector('md-radio-button.md-checked');
        var target = buttons[increment < 0 ? 'previous' : 'next'](selected, validate) || buttons.first();
  
        // Activate radioButton's click listener (triggerHandler won't create a real click event)
        angular.element(target).triggerHandler('click');
      }
    }
docs: Change the radio group's selected button by a given increment.
  If no button is selected, select the first button.