**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 (!child || getTreeScope(self) !== getTreeScope(child)) return false;
      for (var node = child; node; node = node.parentNode) {
        if (node === self) return true;
      }
      return false;
    }
    var OriginalNode = window.Node;
    function Node(original) {
      assert(original instanceof OriginalNode);
      EventTarget.call(this, original);
      this.parentNode_ = undefined;
      this.firstChild_ = undefined;
      this.lastChild_ = undefined;
      this.nextSibling_ = undefined;
      this.previousSibling_ = undefined;
      this.treeScope_ = undefined;
    }
    var OriginalDocumentFragment = window.DocumentFragment;
    var originalAppendChild = OriginalNode.prototype.appendChild;
    var originalCompareDocumentPosition = OriginalNode.prototype.compareDocumentPosition;
    var originalInsertBefore = OriginalNode.prototype.insertBefore;
    var originalRemoveChild = OriginalNode.prototype.removeChild;
    var originalReplaceChild = OriginalNode.prototype.replaceChild;
    var isIe = /Trident/.test(navigator.userAgent);
    var removeChildOriginalHelper = isIe ? function(parent, child) {
      try {
        originalRemoveChild.call(parent, child);
      } catch (ex) {
        if (!(parent instanceof OriginalDocumentFragment)) throw ex;