**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 e = node.firstElementChild;
    if (!e) {
      e = node.firstChild;
      while (e && e.nodeType !== Node.ELEMENT_NODE) {
        e = e.nextSibling;
      }
    }
    while (e) {
      if (find(e, data) !== true) {
        findAllElements(e, find, data);
      }
      e = e.nextElementSibling;
    }
    return null;
  }
  function forRoots(node, cb) {
    var root = node.shadowRoot;
    while (root) {
      forSubtree(root, cb);
      root = root.olderShadowRoot;
    }
  }
  var processingDocuments;
  function forDocumentTree(doc, cb) {