fix(select): invalidate when 'multiple, required and model is []`

When `multiple` attribute is set on a `<select>` control and the model value is an empty array,
we should invalidate the control.  Previously, this directive was using incorrect logic for
determining if the model was empty.

Closes angular#5337

src/ng/directive/select.js

@@ -221,18 +221,10 @@ var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
       selectCtrl.init(ngModelCtrl, nullOption, unknownOption);
 
       // required validator
-      if (multiple && (attr.required || attr.ngRequired)) {
-        var requiredValidator = function(value) {
-          ngModelCtrl.$setValidity('required', !attr.required || (value && value.length));
-          return value;
+      if (multiple) {
+        ngModelCtrl.$isEmpty = function(value) {
+          return !value || value.length === 0;
         };
-
-        ngModelCtrl.$parsers.push(requiredValidator);
-        ngModelCtrl.$formatters.unshift(requiredValidator);
-
-        attr.$observe('required', function() {
-          requiredValidator(ngModelCtrl.$viewValue);
-        });
       }
 
       if (optionsExp) setupAsOptions(scope, element, ngModelCtrl);

test/ng/directive/selectSpec.js

@@ -1215,6 +1215,30 @@ describe('select', function() {
       });
 
 
+      it('should treat an empty array as invalid when `multiple` attribute used', function() {
+        createSelect({
+          'ng-model': 'value',
+          'ng-options': 'item.name for item in values',
+          'ng-required': 'required',
+          'multiple': ''
+        }, true);
+
+        scope.$apply(function() {
+          scope.value = [];
+          scope.values = [{name: 'A', id: 1}, {name: 'B', id: 2}];
+          scope.required = true;
+        });
+        expect(element).toBeInvalid();
+
+        scope.$apply(function() {
+          // ngModelWatch does not set objectEquality flag
+          // array must be replaced in order to trigger $formatters
+          scope.value = [scope.values[0]];
+        });
+        expect(element).toBeValid();
+      });
+
+
       it('should allow falsy values as values', function() {
         createSelect({
           'ng-model': 'value',