How to unit test custom decorator with Jasmine

So I have such decorator in app config:

    angular.module('app').config(['$provide', function ($provide) {

    $provide.decorator('$rootScope', ['$delegate', function ($delegate) {
        $delegate.constructor.prototype.$onRootScope = function (name, listener) {
            var unsubscribe = $delegate.$on(name, listener);
            this.$on('$destroy', unsubscribe);
        };

        $delegate.constructor.prototype.$watchRootScope = function (name, listener) {
            var unsubscribe = $delegate.$watch(name, listener);
            this.$on('$destroy', unsubscribe);
        };

        $delegate.constructor.prototype.$watchAfterLoad = function (watchExpression, listener, objectEquality) {
            var initialLoad = true;
            this.$watch(watchExpression, function () {
                if (initialLoad) {
                    // note: this obviously runs outside of angular, so sometimes the timeout could run after initial load
                    setTimeout(function () { initialLoad = false; }, 25);
                } else {
                    listener.apply(this, arguments);
                }
            }, objectEquality);
        };

        return $delegate;
    }]);

}]);

As you can see this decorator lets me use $scope.$onRootScope instead of $rootScope.$on and takes care of automatic listeners removal on scope destroy event...

When I unit test my code which logic contains $scope.$onRootScope I'm getting such error: TypeError: undefined is not a constructor (evaluating 'scope.$onRootScope') in

Before each test I'm loading all required models and do inject which looks like this ~:

beforeEach(function () {
    inject(function (_$rootScope_) {
        $rootScope = _$rootScope_;
    });
});

How should I overcome this problem? Is there a way to mock / mimic $scope.$onRootScope behaviour?

I'm quite new to unit testing & Jasmine so sorry for not very nicely formatted question.