The marker symbol is used to define the symbol for features associated with a popup window. In this test application, no features are selected so the marker symbol does not display. If the application contains features that are associated with a popup window, use the following code to modify the marker symbol:
map.infoWindow.set("markerSymbol", new SimpleMarkerSymbol());
Documentation for this can be found in the Popup API reference: https://developers.arcgis.com/javascript/jsapi/popup-amd.html