Returns the display value for an item.
@param item
@returns {*}
function getDisplayValue (item) {
    return $q.when(getItemText(item) || item).then(function(itemText) {
      if (itemText && !angular.isString(itemText)) {
        $log.warn('md-autocomplete: Could not resolve display value to a string. ' +
          'Please check the `md-item-text` attribute.');
      }

      return itemText;
    });

    /**
     * Getter function to invoke user-defined expression (in the directive)
     * to convert your object to a single string.
     */
    function getItemText (item) {
      return (item && $scope.itemText) ? $scope.itemText(getItemAsNameVal(item)) : null;
    }
  }