fix(NgModel): make sure the required validator uses the $validators pipeline

Fixes angular#5164

Author: matsko

src/ng/directive/input.js

@@ -1781,6 +1781,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
    */
   this.$commitViewValue = function() {
     var viewValue = ctrl.$viewValue;
+
     $timeout.cancel(pendingDebounce);
     if (ctrl.$$lastCommittedViewValue === viewValue) {
       return;
@@ -2130,21 +2131,12 @@ var requiredDirective = function() {
       if (!ctrl) return;
       attr.required = true; // force truthy in case we are on non input element
 
-      var validator = function(value) {
-        if (attr.required && ctrl.$isEmpty(value)) {
-          ctrl.$setValidity('required', false);
-          return;
-        } else {
-          ctrl.$setValidity('required', true);
-          return value;
-        }
+      ctrl.$validators.required = function(modelValue, viewValue) {
+        return !attr.required || !ctrl.$isEmpty(viewValue);
       };
 
-      ctrl.$formatters.push(validator);
-      ctrl.$parsers.unshift(validator);
-
       attr.$observe('required', function() {
-        validator(ctrl.$viewValue);
+        ctrl.$validate();
       });
     }
   };

test/ng/directive/inputSpec.js

@@ -2564,7 +2564,7 @@ describe('input', function() {
       compileInput('<input type="text" ng-model="name" name="alias" required />');
 
       scope.$apply(function() {
-        scope.name = '';
+        scope.name = null;
       });
 
       expect(inputElm).toBeInvalid();