Add console.log shim to avoid errors while deving.

test/asset/worker.js

@@ -1,9 +1,12 @@
+if (!self.console) {
+  console = { 'log': function() {} };
+}
 addEventListener('message', function(e) {
   if (e.data) {
     try {
       importScripts('../' + e.data);
     } catch(e) {
-      self._ = {};
+      self._ = { 'VERSION': e.message };
     }
     postMessage(_.VERSION);
   }