**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

/**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @providesModule ReactDOMFiberEntry
 * @flow
 */
'use strict';
import type {ReactNodeList} from 'ReactTypes';
require('checkReact');
var DOMNamespaces = require('DOMNamespaces');
var ExecutionEnvironment = require('fbjs/lib/ExecutionEnvironment');
var ReactBrowserEventEmitter = require('ReactBrowserEventEmitter');
var ReactControlledComponent = require('ReactControlledComponent');
var ReactDOMComponentTree = require('ReactDOMComponentTree');
var ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
var ReactDOMFiberComponent = require('ReactDOMFiberComponent');
var ReactDOMFrameScheduling = require('ReactDOMFrameScheduling');
var ReactGenericBatching = require('ReactGenericBatching');
var ReactFiberReconciler = require('ReactFiberReconciler');
var ReactInputSelection = require('ReactInputSelection');
var ReactInstanceMap = require('ReactInstanceMap');
var ReactPortal = require('ReactPortal');
var ReactVersion = require('ReactVersion');
var {ReactCurrentOwner} = require('ReactGlobalSharedState');
var {injectInternals} = require('ReactFiberDevToolsHook');