Support serving ES6 JS to clients (#596)

* Support serving ES6 JS to clients

* Make es6 default dir and es5 a special dir

* Fix package building

* Fix build_frontend script

gulp/service-worker-dev.js.tmpl

@@ -0,0 +1,8 @@
+console.warn('Service worker caching disabled in development');
+self.addEventListener('install', function(event) {
+  self.skipWaiting();
+});
+
+self.addEventListener('activate', function(event) {
+  self.clients.claim();
+});