**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

};
  }
})(window.WebComponents);
(function(global) {
  var registrationsTable = new WeakMap();
  var setImmediate;
  if (/Trident/.test(navigator.userAgent)) {
    setImmediate = setTimeout;
  } else if (window.setImmediate) {
    setImmediate = window.setImmediate;
  } else {
    var setImmediateQueue = [];
    var sentinel = String(Math.random());
    window.addEventListener("message", function(e) {
      if (e.data === sentinel) {
        var queue = setImmediateQueue;
        setImmediateQueue = [];
        queue.forEach(function(func) {
          func();
        });
      }
    });
    setImmediate = function(func) {
      setImmediateQueue.push(func);
      window.postMessage(sentinel, "*");
    };
  }
  var isScheduled = false;
  var scheduledObservers = [];
  function scheduleCallback(observer) {