Use require.extensions to replace .client imports

The simplest server doesn't need AOT compilation. Instead we can just
configure require.extensions. This is probably not the best idea to use
in prod but is enough to show the set up.

fixtures/flight/server/handler.server.js

@@ -8,12 +8,12 @@ module.exports = function(req, res) {
   res.setHeader('Access-Control-Allow-Origin', '*');
   pipeToNodeWritable(<App />, res, {
     // TODO: Read from a map on the disk.
-    './src/Counter.client.js': {
+    [require.resolve('../src/Counter.client.js')]: {
       id: './src/Counter.client.js',
       chunks: ['1'],
       name: 'default',
     },
-    './src/ShowMore.client.js': {
+    [require.resolve('../src/ShowMore.client.js')]: {
       id: './src/ShowMore.client.js',
       chunks: ['2'],
       name: 'default',

fixtures/flight/server/index.js

@@ -1,5 +1,12 @@
 'use strict';
 
+require.extensions['.client.js'] = function(module, path) {
+  module.exports = {
+    $$typeof: Symbol.for('react.module.reference'),
+    name: path,
+  };
+};
+
 const babelRegister = require('@babel/register');
 
 babelRegister({

fixtures/flight/src/App.server.js

@@ -2,16 +2,9 @@ import * as React from 'react';
 
 import Container from './Container';
 
-// TODO: A transform should read this from webpack plugin output.
-const Counter = {
-  $$typeof: Symbol.for('react.module.reference'),
-  name: './src/Counter.client.js',
-};
+import Counter from './Counter.client';
 
-const ShowMore = {
-  $$typeof: Symbol.for('react.module.reference'),
-  name: './src/ShowMore.client.js',
-};
+import ShowMore from './ShowMore.client';
 
 export default function App() {
   return (