**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

scheduledObservers.push(observer);
    if (!isScheduled) {
      isScheduled = true;
      setImmediate(dispatchCallbacks);
    }
  }
  function wrapIfNeeded(node) {
    return window.ShadowDOMPolyfill && window.ShadowDOMPolyfill.wrapIfNeeded(node) || node;
  }
  function dispatchCallbacks() {
    isScheduled = false;
    var observers = scheduledObservers;
    scheduledObservers = [];
    observers.sort(function(o1, o2) {
      return o1.uid_ - o2.uid_;
    });
    var anyNonEmpty = false;
    observers.forEach(function(observer) {
      var queue = observer.takeRecords();
      removeTransientObserversFor(observer);
      if (queue.length) {
        observer.callback_(queue, observer);
        anyNonEmpty = true;
      }
    });
    if (anyNonEmpty) dispatchCallbacks();
  }
  function removeTransientObserversFor(observer) {