**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

var relatedTarget = relatedTargetTable.get(this);
        if (relatedTarget !== undefined) return relatedTarget;
        return wrap(unwrap(this).relatedTarget);
      }
    };
    function getInitFunction(name, relatedTargetIndex) {
      return function() {
        arguments[relatedTargetIndex] = unwrap(arguments[relatedTargetIndex]);
        var impl = unwrap(this);
        impl[name].apply(impl, arguments);
      };
    }
    var mouseEventProto = mixin({
      initMouseEvent: getInitFunction("initMouseEvent", 14)
    }, relatedTargetProto);
    var focusEventProto = mixin({
      initFocusEvent: getInitFunction("initFocusEvent", 5)
    }, relatedTargetProto);
    var MouseEvent = registerGenericEvent("MouseEvent", UIEvent, mouseEventProto);
    var FocusEvent = registerGenericEvent("FocusEvent", UIEvent, focusEventProto);
    var defaultInitDicts = Object.create(null);
    var supportsEventConstructors = function() {
      try {
        new window.FocusEvent("focus");
      } catch (ex) {
        return false;
      }
      return true;
    }();
    function constructEvent(OriginalEvent, name, type, options) {