feat(ngModel): provide ng-empty and ng-not-empty CSS classes

If the `$viewValue` is empty then the `ng-empty` CSS class is applied
to the input. Conversely, if it is not empty the `ng-not-empty` CSS class
is applied. Emptiness is ascertained by calling `NgModelController.$isEmpty()`

Closes #10050

Author: petebacondarwin

src/ng/directive/ngModel.js

@@ -319,7 +319,7 @@ var NgModelController = ['$scope', '$exceptionHandler', '$attrs', '$element', '$
   };
 
   this.$$updateEmptyClasses = function(value) {
-    if (this.$isEmpty(value)) {
+    if (ctrl.$isEmpty(value)) {
       $animate.removeClass($element, NOT_EMPTY_CLASS);
       $animate.addClass($element, EMPTY_CLASS);
     } else {

test/ng/directive/ngModelSpec.js

@@ -1328,7 +1328,9 @@ describe('ngModel', function() {
   describe('CSS classes', function() {
     var EMAIL_REGEXP = /^[a-z0-9!#$%&'*+\/=?^_`{|}~.-]+@[a-z0-9]([a-z0-9-]*[a-z0-9])?(\.[a-z0-9]([a-z0-9-]*[a-z0-9])?)*$/i;
 
-    it('should set ng-empty or ng-not-empty the view value changes', inject(function($compile, $rootScope, $sniffer) {
+    it('should set ng-empty or ng-not-empty when the view value changes',
+          inject(function($compile, $rootScope, $sniffer) {
+
       var element = $compile('<input ng-model="value" />')($rootScope);
 
       $rootScope.$digest();