**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

if (
    !foundDevTools &&
    ExecutionEnvironment.canUseDOM &&
    window.top === window.self
  ) {
    
    if (
      (navigator.userAgent.indexOf('Chrome') > -1 &&
        navigator.userAgent.indexOf('Edge') === -1) ||
      navigator.userAgent.indexOf('Firefox') > -1
    ) {
      const protocol = window.location.protocol;
      
      if (/^(https?|file):$/.test(protocol)) {
        console.info(
          '%cDownload the React DevTools ' +
            'for a better development experience: ' +
            'https://fb.me/react-devtools' +
            (protocol === 'file:'
              ? '\nYou might need to use a local HTTP server (instead of file://): ' +
                  'https://fb.me/react-devtools-faq'
              : ''),
          'font-weight:bold',
        );
      }
    }
  }
}
module.exports = ReactDOMFiber;