**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 (node instanceof DocumentFragment) {
        var nodes = collectNodesForDocumentFragment(node);
        surpressMutations = true;
        for (var i = nodes.length - 1; i >= 0; i--) {
          node.removeChild(nodes[i]);
          nodes[i].parentNode_ = parentNode;
        }
        surpressMutations = false;
        for (var i = 0; i < nodes.length; i++) {
          nodes[i].previousSibling_ = nodes[i - 1] || previousNode;
          nodes[i].nextSibling_ = nodes[i + 1] || nextNode;
        }
        if (previousNode) previousNode.nextSibling_ = nodes[0];
        if (nextNode) nextNode.previousSibling_ = nodes[nodes.length - 1];
        return nodes;
      }
      var nodes = createOneElementNodeList(node);
      var oldParent = node.parentNode;
      if (oldParent) {
        oldParent.removeChild(node);
      }
      node.parentNode_ = parentNode;
      node.previousSibling_ = previousNode;
      node.nextSibling_ = nextNode;
      if (previousNode) previousNode.nextSibling_ = node;
      if (nextNode) nextNode.previousSibling_ = node;
      return nodes;
    }
    function collectNodesNative(node) {