Add missing source attribution in xyz-esri example

examples/xyz-esri.js

@@ -1,3 +1,4 @@
+goog.require('ol.Attribution');
 goog.require('ol.Map');
 goog.require('ol.RendererHint');
 goog.require('ol.View2D');
@@ -6,11 +7,17 @@ goog.require('ol.proj');
 goog.require('ol.source.XYZ');
 
 
+var attribution = new ol.Attribution({
+  html: 'Tiles &copy; <a href="http://services.arcgisonline.com/ArcGIS/' +
+      'rest/services/World_Topo_Map/MapServer">ArcGIS</a>'
+});
+
 var map = new ol.Map({
   target: 'map',
   layers: [
     new ol.layer.Tile({
       source: new ol.source.XYZ({
+        attributions: [attribution],
         url: 'http://server.arcgisonline.com/ArcGIS/rest/services/' +
             'World_Topo_Map/MapServer/tile/{z}/{y}/{x}'
       })