**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 p = node.parentNode;
      return p && p.invalidateShadowRenderer();
    }
    function cleanupNodes(nodes) {
      for (var i = 0, n; i < nodes.length; i++) {
        n = nodes[i];
        n.parentNode.removeChild(n);
      }
    }
    var originalImportNode = document.importNode;
    var originalCloneNode = window.Node.prototype.cloneNode;
    function cloneNode(node, deep, opt_doc) {
      var clone;
      if (opt_doc) clone = wrap(originalImportNode.call(opt_doc, unsafeUnwrap(node), false)); else clone = wrap(originalCloneNode.call(unsafeUnwrap(node), false));
      if (deep) {
        for (var child = node.firstChild; child; child = child.nextSibling) {
          clone.appendChild(cloneNode(child, true, opt_doc));
        }
        if (node instanceof wrappers.HTMLTemplateElement) {
          var cloneContent = clone.content;
          for (var child = node.content.firstChild; child; child = child.nextSibling) {
            cloneContent.appendChild(cloneNode(child, true, opt_doc));
          }
        }
      }
      return clone;
    }
    function contains(self, child) {