removeFromGroupOpen = function() {
      if (self.config.groupName) {
        var index;
        self.config.groupName = coerceToArray(self.config.groupName);
        angular.forEach(self.config.groupName, function(group) {
          group = self._$mdPanel._groups[group];
          index = group.openPanels.indexOf(self);
          if (index > -1) {
            group.openPanels.splice(index, 1);
          }
        });
      }
    }
Hides the panel.
@returns {!Q.IPromise<!MdPanelRef>} A promise that is resolved when
    the panel has hidden and animations finish.