**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

this.addElementToDocument(elt);
    },
    rootImportForElement: function(elt) {
      var n = elt;
      while (n.ownerDocument.__importLink) {
        n = n.ownerDocument.__importLink;
      }
      return n;
    },
    addElementToDocument: function(elt) {
      var port = this.rootImportForElement(elt.__importElement || elt);
      var l = port.__insertedElements = port.__insertedElements || 0;
      var refNode = port.nextElementSibling;
      for (var i = 0; i < l; i++) {
        refNode = refNode && refNode.nextElementSibling;
      }
      port.parentNode.insertBefore(elt, refNode);
    },
    trackElement: function(elt, callback) {
      var self = this;
      var done = function(e) {
        if (callback) {
          callback(e);
        }
        self.markParsingComplete(elt);
        self.parseNext();
      };
      elt.addEventListener("load", done);
      elt.addEventListener("error", done);
      if (isIE && elt.localName === "style") {