use webpack for transpiling too

server/build/bundle.js

-import { resolve, dirname, basename } from 'path'
-import webpack from 'webpack'
-
-export default function bundle (src, dst) {
-  const compiler = webpack({
-    entry: src,
-    output: {
-      path: dirname(dst),
-      filename: basename(dst),
-      libraryTarget: 'commonjs2'
-    },
-    externals: [
-      'react',
-      'react-dom',
-      {
-        [require.resolve('react')]: 'react',
-        [require.resolve('../../lib/link')]: 'next/link',
-        [require.resolve('../../lib/css')]: 'next/css',
-        [require.resolve('../../lib/head')]: 'next/head'
-      }
-    ],
-    resolveLoader: {
-      root: resolve(__dirname, '..', '..', 'node_modules')
-    },
-    plugins: [
-      new webpack.optimize.UglifyJsPlugin({
-        compress: { warnings: false },
-        sourceMap: false
-      })
-    ],
-    module: {
-      preLoaders: [
-        { test: /\.json$/, loader: 'json-loader' }
-      ]
-    }
-  })
-
-  return new Promise((resolve, reject) => {
-    compiler.run((err, stats) => {
-      if (err) return reject(err)
-
-      const jsonStats = stats.toJson()
-      if (jsonStats.errors.length > 0) {
-        const error = new Error(jsonStats.errors[0])
-        error.errors = jsonStats.errors
-        error.warnings = jsonStats.warnings
-        return reject(error)
-      }
-
-      resolve()
-    })
-  })
-}

server/build/index.js

-import { resolve } from 'path'
-import glob from 'glob-promise'
-import transpile from './transpile'
-import bundle from './bundle'
+import webpack from './webpack'
 
 export default async function build (dir) {
-  const dstDir = resolve(dir, '.next')
-  const templateDir = resolve(__dirname, '..', '..', 'pages')
+  const compiler = await webpack(dir)
 
-  // create `.next/pages/_error.js`
-  // which may be overwriten by the user sciprt, `pages/_error.js`
-  const templatPaths = await glob('**/*.js', { cwd: templateDir })
-  await Promise.all(templatPaths.map(async (p) => {
-    await transpile(resolve(templateDir, p), resolve(dstDir, 'pages', p))
-  }))
+  return new Promise((resolve, reject) => {
+    compiler.run((err, stats) => {
+      if (err) return reject(err)
 
-  const paths = await glob('**/*.js', { cwd: dir, ignore: 'node_modules/**' })
-  await Promise.all(paths.map(async (p) => {
-    await transpile(resolve(dir, p), resolve(dstDir, p))
-  }))
+      const jsonStats = stats.toJson()
+      if (jsonStats.errors.length > 0) {
+        const error = new Error(jsonStats.errors[0])
+        error.errors = jsonStats.errors
+        error.warnings = jsonStats.warnings
+        return reject(error)
+      }
 
-  const pagePaths = await glob('pages/**/*.js', { cwd: dstDir })
-  await Promise.all(pagePaths.map(async (p) => {
-    await bundle(resolve(dstDir, p), resolve(dstDir, '_bundles', p))
-  }))
+      resolve()
+    })
+  })
 }

server/build/transpile.js

-import { dirname } from 'path'
-import fs from 'mz/fs'
-import mkdirp from 'mkdirp-then';
-import { transformFile } from 'babel-core'
-import preset2015 from 'babel-preset-es2015'
-import presetReact from 'babel-preset-react'
-import transformAsyncToGenerator from 'babel-plugin-transform-async-to-generator'
-import transformClassProperties from 'babel-plugin-transform-class-properties'
-import transformObjectRestSpread from 'babel-plugin-transform-object-rest-spread'
-import transformRuntime from 'babel-plugin-transform-runtime'
-import moduleAlias from 'babel-plugin-module-alias'
-
-const babelRuntimePath = require.resolve('babel-runtime/package')
-.replace(/[\\\/]package\.json$/, '');
-
-const babelOptions = {
-  presets: [preset2015, presetReact],
-  plugins: [
-    transformAsyncToGenerator,
-    transformClassProperties,
-    transformObjectRestSpread,
-    transformRuntime,
-    [
-      moduleAlias,
-      [
-        { src: `npm:${babelRuntimePath}`, expose: 'babel-runtime' },
-        { src: `npm:${require.resolve('react')}`, expose: 'react' },
-        { src: `npm:${require.resolve('../../lib/link')}`, expose: 'next/link' },
-        { src: `npm:${require.resolve('../../lib/css')}`, expose: 'next/css' },
-        { src: `npm:${require.resolve('../../lib/head')}`, expose: 'next/head' }
-      ]
-    ]
-  ],
-  ast: false
-}
-
-export default async function transpile (src, dst) {
-  const code = await new Promise((resolve, reject) => {
-    transformFile(src, babelOptions, (err, result) => {
-      if (err) return reject(err)
-      resolve(result.code)
-    })
-  })
-
-  await writeFile(dst, code)
-}
-
-async function writeFile (path, data) {
-  await mkdirp(dirname(path))
-  await fs.writeFile(path, data, { encoding: 'utf8', flag: 'w+' })
-}