**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 (cssText) {
        getSheet().appendChild(document.createTextNode(cssText));
      }
    }
    function addOwnSheet(cssText, name) {
      var style = cssTextToStyle(cssText);
      style.setAttribute(name, "");
      style.setAttribute(SHIMMED_ATTRIBUTE, "");
      document.head.appendChild(style);
    }
    var SHIM_ATTRIBUTE = "shim-shadowdom";
    var SHIMMED_ATTRIBUTE = "shim-shadowdom-css";
    var NO_SHIM_ATTRIBUTE = "no-shim";
    var sheet;
    function getSheet() {
      if (!sheet) {
        sheet = document.createElement("style");
        sheet.setAttribute(SHIMMED_ATTRIBUTE, "");
        sheet[SHIMMED_ATTRIBUTE] = true;
      }
      return sheet;
    }
    if (window.ShadowDOMPolyfill) {
      addCssToDocument("style { display: none !important; }\n");
      var doc = ShadowDOMPolyfill.wrap(document);
      var head = doc.querySelector("head");