**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

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 * subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
 * @providesModule WebComponents
 */
window.WebComponents = window.WebComponents || {};
(function(scope) {
  var flags = scope.flags || {};
  var file = "webcomponents.js";
  var script = document.querySelector('script[src*="' + file + '"]');
  var flags = {};
  if (!flags.noOpts) {
    location.search.slice(1).split("&").forEach(function(o) {
      o = o.split("=");
      o[0] && (flags[o[0]] = o[1] || true);
    });
    if (script) {
      for (var i = 0, a; a = script.attributes[i]; i++) {
        if (a.name !== "src") {
          flags[a.name] = a.value || true;
        }
      }
    }
    if (flags.log) {
      var parts = flags.log.split(",");
      flags.log = {};
      parts.forEach(function(f) {
        flags.log[f] = true;
      });
    } else {