Add ol.source.WMTS#getUrls

src/ol/tileurlfunction.js

@@ -2,6 +2,7 @@ goog.provide('ol.TileUrlFunction');
 goog.provide('ol.TileUrlFunctionType');
 
 goog.require('goog.array');
+goog.require('goog.asserts');
 goog.require('goog.math');
 goog.require('ol.TileCoord');
 goog.require('ol.tilecoord');
@@ -74,6 +75,8 @@ ol.TileUrlFunction.createFromTemplates = function(templates) {
  * @return {ol.TileUrlFunctionType} Tile URL function.
  */
 ol.TileUrlFunction.createFromTileUrlFunctions = function(tileUrlFunctions) {
+  goog.asserts.assert(tileUrlFunctions.length > 0,
+      'Length of tile url functions should be greater than 0');
   if (tileUrlFunctions.length === 1) {
     return tileUrlFunctions[0];
   }