browser.ignoreSynchronization = true;
showNotificationButton.click();
// check if notification appears
browser.executeAsyncScript(clearAllNotifications);
browser.ignoreSynchronization = false;
// test runs and go ahead immediately

function clearAllNotifications(callback) {
    (function() {
        var el = document.querySelector('[ng-app]'); 
        var injector = angular.element(el).injector();
        var Notification = injector.get('Notification');
        var $timeout = injector.get('$timeout');
        Notification.clearAll();
        callback();
    })();
}