**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

doc = doc || rootDocument;
    whenDocumentReady(function() {
      watchImportsLoad(callback, doc);
    }, doc);
  }
  var requiredReadyState = isIE ? "complete" : "interactive";
  var READY_EVENT = "readystatechange";
  function isDocumentReady(doc) {
    return doc.readyState === "complete" || doc.readyState === requiredReadyState;
  }
  function whenDocumentReady(callback, doc) {
    if (!isDocumentReady(doc)) {
      var checkReady = function() {
        if (doc.readyState === "complete" || doc.readyState === requiredReadyState) {
          doc.removeEventListener(READY_EVENT, checkReady);
          whenDocumentReady(callback, doc);
        }
      };
      doc.addEventListener(READY_EVENT, checkReady);
    } else if (callback) {
      callback();
    }
  }
  function markTargetLoaded(event) {
    event.target.__loaded = true;
  }
  function watchImportsLoad(callback, doc) {