**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

},
    fetch: function(url, elt) {
      flags.load && console.log("fetch", url, elt);
      if (url.match(/^data:/)) {
        var pieces = url.split(",");
        var header = pieces[0];
        var body = pieces[1];
        if (header.indexOf(";base64") > -1) {
          body = atob(body);
        } else {
          body = decodeURIComponent(body);
        }
        setTimeout(function() {
          this.receive(url, elt, null, body);
        }.bind(this), 0);
      } else {
        var receiveXhr = function(err, resource, redirectedUrl) {
          this.receive(url, elt, err, resource, redirectedUrl);
        }.bind(this);
        xhr.load(url, receiveXhr);
      }
    },
    receive: function(url, elt, err, resource, redirectedUrl) {
      this.cache[url] = resource;
      var $p = this.pending[url];
      for (var i = 0, l = $p.length, p; i < l && (p = $p[i]); i++) {
        this.onload(url, p, resource, err, redirectedUrl);
        this.tail();
      }
      this.pending[url] = null;