Added $asyncValidators

Check https://docs.angularjs.org/api/ng/type/ngModel.NgModelController#$asyncValidators

angularjs/angular.d.ts

@@ -445,6 +445,7 @@ declare module ng {
         $untouched: boolean;
 
         $validators: IModelValidators;
+        $asyncValidators: IModelValidators;
 
         $pristine: boolean;
         $dirty: boolean;