fix(directive): reapply ng-class when interpolated class attribute changes

Closes angular#1016

Author: maxmart

src/ng/compile.js

@@ -999,9 +999,7 @@ function $CompileProvider($provide) {
           // we define observers array only for interpolated attrs
           // and ignore observers for non interpolated attrs to save some memory
           attr.$$observers[name] = [];
-          if (name !== 'class') {
-        	  attr[name] = undefined;
-          }
+          attr[name] = undefined;
           scope.$watch(interpolateFn, function(value) {
             attr.$set(name, value);
           });

src/ng/directive/ngClass.js

@@ -2,35 +2,27 @@
 
 function classDirective(name, selector) {
   name = 'ngClass' + name;
-  return ['$interpolate', function($interpolate) {
-    return function(scope, element, attr) {
-      // Reusable function for re-applying the ngClass
-      function reapply(newVal, oldVal) {
-        if (selector === true || scope.$index % 2 === selector) {
-          if (oldVal && (newVal !== oldVal)) {
-            if (isObject(oldVal) && !isArray(oldVal))
-              oldVal = map(oldVal, function(v, k) { if (v) return k });
-            element.removeClass(isArray(oldVal) ? oldVal.join(' ') : oldVal);
-          }
-          if (isObject(newVal) && !isArray(newVal))
-            newVal = map(newVal, function(v, k) { if (v) return k });
-          if (newVal) element.addClass(isArray(newVal) ? newVal.join(' ') : newVal);
+  return ngDirective(function(scope, element, attr) {
+    // Reusable function for re-applying the ngClass
+    function reapply(newVal, oldVal) {
+      if (selector === true || scope.$index % 2 === selector) {
+        if (oldVal && (newVal !== oldVal)) {
+          if (isObject(oldVal) && !isArray(oldVal))
+            oldVal = map(oldVal, function(v, k) { if (v) return k });
+          element.removeClass(isArray(oldVal) ? oldVal.join(' ') : oldVal);
         }
-      };
-      scope.$watch(attr[name], reapply, true);
-      
-      // Watch class attribute for changes
-      if (attr['class']) {
-        var interpolateFn = $interpolate(attr['class'], true);
-        scope.$watch(interpolateFn, function(newClass, oldClass) {
-          // Reapply ngClass
-          var ngClass = scope.$eval(attr[name]);
-          reapply(ngClass, ngClass);
-        });
+        if (isObject(newVal) && !isArray(newVal))
+          newVal = map(newVal, function(v, k) { if (v) return k });
+        if (newVal) element.addClass(isArray(newVal) ? newVal.join(' ') : newVal);
       }
-
     };
-  }];
+    scope.$watch(attr[name], reapply, true);
+    
+    attr.$observe('class', function(value) {
+      var ngClass = scope.$eval(attr[name]);
+      reapply(ngClass, ngClass);
+    });
+  });
 }
 
 /**