**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

var m, el = node.firstElementChild;
      while (el) {
        if (el.matches(selector)) return el;
        m = findOne(el, selector);
        if (m) return m;
        el = el.nextElementSibling;
      }
      return null;
    }
    function matchesSelector(el, selector) {
      return el.matches(selector);
    }
    var XHTML_NS = "http://www.w3.org/1999/xhtml";
    function matchesTagName(el, localName, localNameLowerCase) {
      var ln = el.localName;
      return ln === localName || ln === localNameLowerCase && el.namespaceURI === XHTML_NS;
    }
    function matchesEveryThing() {
      return true;
    }
    function matchesLocalNameOnly(el, ns, localName) {
      return el.localName === localName;
    }
    function matchesNameSpace(el, ns) {
      return el.namespaceURI === ns;
    }
    function matchesLocalNameNS(el, ns, localName) {
      return el.namespaceURI === ns && el.localName === localName;
    }
    function findElements(node, index, result, p, arg0, arg1) {