fix(ngModel): prevent internal scope reference from being copied

Fixes #15833

Author: jbedard

src/ng/directive/ngModel.js

@@ -281,7 +281,9 @@ function NgModelController($scope, $exceptionHandler, $attr, $element, $parse, $
 
   this.$$currentValidationRunId = 0;
 
-  this.$$scope = $scope;
+  // https://github.com/angular/angular.js/issues/15833
+  // Prevent `$$scope` from being iterated over by `copy` when NgModelController is deep watched
+  Object.defineProperty(this, '$$scope', {value: $scope});
   this.$$attr = $attr;
   this.$$element = $element;
   this.$$animate = $animate;

test/ng/compileSpec.js

@@ -5888,6 +5888,29 @@ describe('$compile', function() {
               expect(componentScope.owRef).toEqual({name: 'lucas', item: {name: 'martin'}});
             }));
 
+            // https://github.com/angular/angular.js/issues/15833
+            it('should work with ng-model inputs', function() {
+              var componentScope;
+
+              module(function($compileProvider) {
+                $compileProvider.directive('undi', function() {
+                  return {
+                    restrict: 'A',
+                    scope: {
+                      undi: '<'
+                    },
+                    link: function($scope) { componentScope = $scope; }
+                  };
+                });
+              });
+
+              inject(function($compile, $rootScope) {
+                element = $compile('<form name="f" undi="[f.i]"><input name="i" ng-model="a"/></form>')($rootScope);
+                $rootScope.$apply();
+                expect(componentScope.undi).toBeDefined();
+              });
+            });
+
 
             it('should not complain when the isolated scope changes', inject(function() {
               compile('<div><span my-component ow-ref="{name: name}">');