code: function build (config) {
    return new Promise((resolve, reject) => {
      const compiler = webpack(Object.assign(config, {
        output: { filename: 'file' },
        plugins: [
          new webpack.DefinePlugin({
            'process.env.NODE_ENV': '"production"',
            'process.browser': true
          })
        ]
      }), (err, stats) => {
        if (err || stats.hasErrors()) reject(err || new Error(stats.toString('errors-only')))
        const compilation = stats.compilation
        const compiler = compilation.compiler
        const memoryFs = compiler.outputFileSystem
        const outputFile = compilation.assets.file.existsAt
        resolve(memoryFs.readFileSync(outputFile, 'utf8'))
      })
      compiler.outputFileSystem = new MemoryFs()
    })
  }
docs: Uses webpack to build a file in memory and return the bundle.
  
  @param {object} config - webpack config options.
  @return {Promise<string>}