webpack+babel+koa2 Error
### I have an application with a server direcotry. index.js and server.js in this direcotry.
index.js
```js
import('babel-register');
import('babel-polyfill');
import('./server.js');
```
as you can see  I replaced the require with import. and in the server.js
```js
import Koa from 'koa';
const app = new Koa();
app.use(async (ctx, next) => {
  const start = new Date();
  await next();
  const ms = new Date() - start;
  console.log(`${ctx.method} ${ctx.url} - ${ms}ms`);
});
// response
app.use((ctx) => {
  ctx.body = 'Hello Koa in app-async.js';
});
app.listen(8080);
console.log("booting up...，port：8080");
module.exports = app;
```

and I have configured the webpack.config.js with this:
```js
const path = require('path');

module.exports = {
  entry:path.resolve(__dirname,'server/index.js'),
  output:{
    path:path.resolve(__dirname,'server'),
    filename:"start.js"
  }
}

module.export = {
  module:{
    rules:[
      {test:/\.js$/,use:['babel-loader'],exclude:'/node_modules/'}
    ]
  }
}
```
and the .babelrc file has these contetts
```js
{
  "presets": [
    ["env", {
      "targets": {
        "node": true
      }
    }]
  ]
}
```
**But**I received this error:
```js
ERROR in ./~/koa/lib/request.js
Module not found: Error: Can't resolve 'net' in '/Users/git/WorkSpace/vcg/test/World/node_modules/koa/lib'
 @ ./~/koa/lib/request.js 9:12-26
 @ ./~/koa/lib/application.js
 @ ./server/server.js
 @ ./server/index.js

ERROR in ./~/destroy/index.js
Module not found: Error: Can't resolve 'fs' in '/Users/git/WorkSpace/vcg/test/World/node_modules/destroy'
 @ ./~/destroy/index.js 14:17-30
 @ ./~/koa/lib/response.js
 @ ./~/koa/lib/application.js
 @ ./server/server.js
 @ ./server/index.js
```
So I think the koa2 not support webpack