**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

} else {
        impl = new OriginalFormData(formElement && unwrap(formElement));
      }
      setWrapper(impl, this);
    }
    registerWrapper(OriginalFormData, FormData, new OriginalFormData());
    scope.wrappers.FormData = FormData;
  })(window.ShadowDOMPolyfill);
  (function(scope) {
    "use strict";
    var unwrapIfNeeded = scope.unwrapIfNeeded;
    var originalSend = XMLHttpRequest.prototype.send;
    XMLHttpRequest.prototype.send = function(obj) {
      return originalSend.call(this, unwrapIfNeeded(obj));
    };
  })(window.ShadowDOMPolyfill);
  (function(scope) {
    "use strict";
    var isWrapperFor = scope.isWrapperFor;
    var elements = {
      a: "HTMLAnchorElement",
      area: "HTMLAreaElement",
      audio: "HTMLAudioElement",
      base: "HTMLBaseElement",
      body: "HTMLBodyElement",
      br: "HTMLBRElement",
      button: "HTMLButtonElement",
      canvas: "HTMLCanvasElement",
      caption: "HTMLTableCaptionElement",
      col: "HTMLTableColElement",