fix(ngMock): attach $injector to $rootElement

Fixes #14022

Closes #14034

src/ngMock/angular-mocks.js

@@ -2089,9 +2089,9 @@ angular.mock.$RAFDecorator = ['$delegate', function($delegate) {
  *
  */
 angular.mock.$RootElementProvider = function() {
-  this.$get = function() {
-    return angular.element('<div ng-app></div>');
-  };
+  this.$get = ['$injector', function($injector) {
+    return angular.element('<div ng-app></div>').data('$injector', $injector);
+  }];
 };
 
 /**

test/ngMock/angular-mocksSpec.js

@@ -1647,6 +1647,10 @@ describe('ngMock', function() {
     it('should create mock application root', inject(function($rootElement) {
       expect($rootElement.text()).toEqual('');
     }));
+
+    it('should attach the `$injector` to `$rootElement`', inject(function($injector, $rootElement) {
+      expect($rootElement.injector()).toBe($injector);
+    }));
   });