**Do you want to request a *feature* or report a *bug*?**
bug

**What is the current behavior?**
React 15.x was able to render within a DocumentFragment/shadowRoot, not possible with Fiber anymore

**DEMO**

https://www.webpackbin.com/bins/-KvPFG7-HGfQ34IgUxQt

```js
import React, {Component} from 'react'
import {render} from 'react-dom'


class MyComponent extends Component{
  render(){
    return <div>Hello From React !</div>
  }
}

class MyElement extends HTMLElement {
  static is = 'my-element'
  constructor(){
    super()
    const shadowRoot = this.attachShadow({mode:'open'})
    try {
      // this fails 
      render(<MyComponent/>, shadowRoot)
    } catch (e){
      console.error(e)
      shadowRoot.innerHTML = `
        <div style="color:red;">${e}</div>
        <div>see console log for stack trace</div>
       `
    }
  }
  connectedCallback(){
    console.log('MyElement mounted!')
  }
}

customElements.define(MyElement.is, MyElement)
```


related -> https://github.com/skatejs/renderer-react/issues/3

**What is the expected behavior?**
be able to render to shadow root like previous versions of React

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
React 16
Chrome

import {parse, stringify} from 'query-string';
import getVersionTags from '../tags';
const React = window.React;
class Header extends React.Component {
  constructor(props, context) {
    super(props, context);
    const query = parse(window.location.search);
    const version = query.version || 'local';
    const versions = [version];
    this.state = {version, versions};
  }
  componentWillMount() {
    getVersionTags().then(tags => {
      let versions = tags.map(tag => tag.name.slice(1));
      versions = [`local`, ...versions];
      this.setState({versions});
    });
  }
  handleVersionChange(event) {
    const query = parse(window.location.search);
    query.version = event.target.value;
    if (query.version === 'local') {
      delete query.version;
    }
    window.location.search = stringify(query);
  }
  handleFixtureChange(event) {