refactor(ngModel): use local scope param in watcher

src/ng/directive/ngModel.js

@@ -890,8 +890,8 @@ function setupModelWatcher(ctrl) {
   //    -> scope value did not change since the last digest as
   //       ng-change executes in apply phase
   // 4. view should be changed back to 'a'
-  ctrl.$$scope.$watch(function ngModelWatch() {
-    var modelValue = ctrl.$$ngModelGet(ctrl.$$scope);
+  ctrl.$$scope.$watch(function ngModelWatch(scope) {
+    var modelValue = ctrl.$$ngModelGet(scope);
 
     // if scope model value and ngModel value are out of sync
     // TODO(perf): why not move this to the action fn?