add an ol.layer.xyz factory

src/api/layer/xyz.js

@@ -0,0 +1,29 @@
+goog.provide('ol.layer.xyz');
+
+/**
+ * @export
+ * @param {Object} opt_arg Config object.
+ * @return {ol.layer.XYZ}
+ */
+ol.layer.xyz = function(opt_arg) {
+    if (opt_arg instanceof ol.layer.XYZ) {
+        return opt_arg;
+    }
+
+    /** @type {string} */
+    var url;
+
+    var usage = 'ol.layer.xyz accepts an object with a "url" property';
+
+    if (goog.isObject(opt_arg)) {
+        url = opt_arg['url'];
+    } else {
+        throw new Error(usage);
+    }
+
+    if (!goog.isDef(url)) {
+        throw new Error(usage);
+    }
+
+    return new ol.layer.XYZ(url);
+};