fix(ngClass): keep track of old ngClass value manually

ngClassWatchAction, when called as a $watch function, gets the wrong old
value after it has been invoked previously due to observation of the
interpolated class attribute. As a result it doesn't remove classes
properly. Keeping track of the old value manually seems to fix this.

Closes #1637

src/ng/directive/ngClass.js

@@ -3,6 +3,7 @@
 function classDirective(name, selector) {
   name = 'ngClass' + name;
   return ngDirective(function(scope, element, attr) {
+    var oldVal = undefined;
 
     scope.$watch(attr[name], ngClassWatchAction, true);
 
@@ -26,13 +27,14 @@ function classDirective(name, selector) {
     }
 
 
-    function ngClassWatchAction(newVal, oldVal) {
+    function ngClassWatchAction(newVal) {
       if (selector === true || scope.$index % 2 === selector) {
         if (oldVal && (newVal !== oldVal)) {
           removeClass(oldVal);
         }
         addClass(newVal);
       }
+      oldVal = newVal;
     }
 
 

test/ng/directive/ngClassSpec.js

@@ -236,6 +236,17 @@ describe('ngClass', function() {
   }));
 
 
+  it('should not mess up class value due to observing an interpolated class attribute', inject(function($rootScope, $compile) {
+    $rootScope.foo = true;
+    $rootScope.$watch("anything", function() {
+      $rootScope.foo = false;
+    });
+    element = $compile('<div ng-class="{foo:foo}"></div>')($rootScope);
+    $rootScope.$digest();
+    expect(element.hasClass('foo')).toBe(false);
+  }));
+
+
   it('should update ngClassOdd/Even when model is changed by filtering', inject(function($rootScope, $compile) {
     element = $compile('<ul>' +
       '<li ng-repeat="i in items" ' +