Convert common files to es2015.

common/script/content/translation.js

@@ -1,20 +1,11 @@
-'use strict';
+import i18n from '../i18n';
 
-var i18n = require('../i18n');
-
-var t = function(string, vars) {
-  var func = function(lang) {
-    if (vars == null) {
-      vars = {
-        a: 'a'
-      };
-    }
+export default function translator (string, vars = { a: 'a' }) {
+  function func (lang) {
     return i18n.t(string, vars, lang);
-  };
+  }
 
   func.i18nLangFunc = true; // Trick to recognize this type of function
 
   return func;
-};
-
-module.exports = t;
+}