我代码如下： index.js

import React, { Component } from 'react';
import ReactDOM from 'react-dom';
import Header from './header';
import './index.scss';

class App extends Component {
    render() {
        return (<Header></Header>);
    }
}
const app = window.document.getElementById('app');
ReactDOM.render(<App />, app)
;
header.js
```js
import React, { Component } from 'react';

export default class Header extends Component {

    render() {
        return (<section className="rheader">
            fuck
        </section>);
    }
}

两个文件中我必须显式import React而且R必须大写，写成小写都不行。否则就报Uncaught ReferenceError: React is not defined。 这是什么原因，是必须这样做还是和babel的配置有关系？我babelrc文件如下

{ "presets": ["es2015", "stage-0","react"],
  "plugins": [
    "add-module-exports",
    "transform-decorators-legacy",
    "transform-class-properties",
    "transform-runtime"]
	}