Fix polyfill module

In require method clear module.exports after importing by null instead of undefined. In that case modules cache would work fine for modules without export

Author: Asondo

fs/polyfill.js

@@ -15,7 +15,7 @@ function require(path) {
     // Add module to cache
     c = module._cache[path] = module.exports;
     // clean exports data to prevent modules duplication
-    module.exports = undefined;
+    module.exports = null;
 
     return c;
 }