title: Cannot call a class as a function, React router show error in console if i use es6 format
body: <p>Im using react router extension to define react routes in my application. every thing in working fine, but the issue is that if i use es6 syntax to define class <code>App.js</code> than <code>index.js</code> "cannot call a class as a function".
  if change <code>App.js</code> file format to <code>App.jsx</code> it resolve my issue.
  How does i can define <code>index.jsx</code> file in <code>.js</code> <code>es6</code> format ? or any proper way to resolve this issue? 
  these are my react router file and app.js file.
  Here is github link of react-router library
  <a href="https://github.com/reactjs/react-router" rel="noreferrer">https://github.com/reactjs/react-router</a></p>
  
  <pre><code>index.jsx
  
  import React from 'react'
  import { render } from 'react-dom'
  import { Router, Route, browserHistory } from 'react-router'
  import App from './app/App'
  
  render((
    &lt;Router history={browserHistory}&gt;
      &lt;Route path="/" component={App}/&gt; 
  
    &lt;/Router&gt;
  ), document.getElementById('app'))
  </code></pre>
  
  <p>Now react router file is 
      //  App.js file</p>
  
  <pre><code>import React from 'react';
  import { Link } from 'react-router'
  
  class App extends React.createClass{
    constructor(props) {
        super(props);
      }
    render() {
      return (
          &lt;div&gt;
           This is a App.js file write in es6.
          &lt;/div&gt;
        )
    }
  }
  
  
  export default App;
  </code></pre>
  
  <p>**Edited: **
  I have following webpack loader config</p>
  
  <pre><code>{
              test:  /(\.js|\.jsx)$/,
              exclude: /node_modules/,
              loader: 'babel',
              query: {
               presets:['es2015','react']
             }
          }
  </code></pre>