go-tour: fix storage when localStorage is not available in the browser.

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/135030043
diff --git a/static/js/services.js b/static/js/services.js
index 344e8b8..3e81a7c 100755
--- a/static/js/services.js
+++ b/static/js/services.js
@@ -60,6 +60,7 @@
     }
 ]).
 
+// Local storage, persistent to page refreshing.
 factory('storage', ['$window',
     function(win) {
         if (win.localStorage) {
@@ -73,8 +74,8 @@
             };
         }
         return {
-            get: function(key, def) {
-                return def;
+            get: function() {
+                return null;
             },
             set: function() {}
         };