fix warning of root metro config (facebook#27883)

Summary:
fix facebook#27620

## Changelog

[Internal] [Fixed] - Fix warning of packager
Pull Request resolved: facebook#27883

Test Plan: - run `./scripts/packager.sh` and no warning displayed

Differential Revision: D19619571

Pulled By: cpojer

fbshipit-source-id: 3cd6383d9b0f7e133a9253b19ad626fcc93d5bed

metro.config.js

@@ -17,8 +17,12 @@ const getPolyfills = require('./rn-get-polyfills');
  * integration tests during local development or on CI services.
  */
 module.exports = {
-  extraNodeModules: {
-    'react-native': __dirname,
+  resolver: {
+    extraNodeModules: {
+      'react-native': __dirname,
+    },
+  },
+  serializer: {
+    getPolyfills,
   },
-  getPolyfills,
 };