**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

if (wrapper instanceof wrappers.ShadowRoot) wrapper = wrapper.host;
      return unwrap(wrapper);
    }
    EventTarget.prototype = {
      addEventListener: function(type, fun, capture) {
        if (!isValidListener(fun) || isMutationEvent(type)) return;
        var listener = new Listener(type, fun, capture);
        var listeners = listenersTable.get(this);
        if (!listeners) {
          listeners = [];
          listeners.depth = 0;
          listenersTable.set(this, listeners);
        } else {
          for (var i = 0; i < listeners.length; i++) {
            if (listener.equals(listeners[i])) return;
          }
        }
        listeners.push(listener);
        var target = getTargetToListenAt(this);
        target.addEventListener_(type, dispatchOriginalEvent, true);
      },
      removeEventListener: function(type, fun, capture) {
        capture = Boolean(capture);
        var listeners = listenersTable.get(this);
        if (!listeners) return;
        var count = 0, found = false;
        for (var i = 0; i < listeners.length; i++) {
          if (listeners[i].type === type && listeners[i].capture === capture) {