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.

Fixes angular#1637

Author: provegard

src/ng/directive/ngClass.js

@@ -26,13 +26,15 @@ function classDirective(name, selector) {
     }
 
 
-    function ngClassWatchAction(newVal, oldVal) {
+    var oldVal = undefined;
+    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

@@ -235,6 +235,15 @@ describe('ngClass', function() {
     expect(element.hasClass('too')).toBeFalsy();
   }));
 
+  it('should not mess up class application due to observing an interpolated class attribute', inject(function($rootScope, $compile) {
+    $rootScope.foo = true;
+    $rootScope.$watch("anything", function() {
+      $rootScope.foo = false;
+    });
+    element = $compile('<div ng-class="{foo:foo}"/>')($rootScope);
+    $rootScope.$digest();
+    expect(element.hasClass('foo')).toBeFalsy();
+  }));
 
   it('should update ngClassOdd/Even when model is changed by filtering', inject(function($rootScope, $compile) {
     element = $compile('<ul>' +