**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

});
(function(scope) {
  var useNative = scope.useNative;
  var initializeModules = scope.initializeModules;
  if (useNative) {
    var nop = function() {};
    scope.watchShadow = nop;
    scope.upgrade = nop;
    scope.upgradeAll = nop;
    scope.upgradeDocumentTree = nop;
    scope.upgradeSubtree = nop;
    scope.takeRecords = nop;
    scope.instanceof = function(obj, base) {
      return obj instanceof base;
    };
  } else {
    initializeModules();
  }
  var upgradeDocumentTree = scope.upgradeDocumentTree;
  if (!window.wrap) {
    if (window.ShadowDOMPolyfill) {
      window.wrap = ShadowDOMPolyfill.wrapIfNeeded;
      window.unwrap = ShadowDOMPolyfill.unwrapIfNeeded;
    } else {
      window.wrap = window.unwrap = function(node) {
        return node;
      };
    }
  }
  function bootstrap() {