**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 {
    var loadEvent = window.HTMLImports && !HTMLImports.ready ? "HTMLImportsLoaded" : "DOMContentLoaded";
    window.addEventListener(loadEvent, bootstrap);
  }
})(window.CustomElements);
(function(scope) {
  if (!Function.prototype.bind) {
    Function.prototype.bind = function(scope) {
      var self = this;
      var args = Array.prototype.slice.call(arguments, 1);
      return function() {
        var args2 = args.slice();
        args2.push.apply(args2, arguments);
        return self.apply(scope, args2);
      };
    };
  }
})(window.WebComponents);
(function(scope) {
  "use strict";
  if (!window.performance) {
    var start = Date.now();
    window.performance = {
      now: function() {
        return Date.now() - start;
      }
    };
  }
  if (!window.requestAnimationFrame) {
    window.requestAnimationFrame = function() {