**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

scope.isWrapper = isWrapper;
    scope.isWrapperFor = isWrapperFor;
    scope.mixin = mixin;
    scope.nativePrototypeTable = nativePrototypeTable;
    scope.oneOf = oneOf;
    scope.registerObject = registerObject;
    scope.registerWrapper = register;
    scope.rewrap = rewrap;
    scope.setWrapper = setWrapper;
    scope.unsafeUnwrap = unsafeUnwrap;
    scope.unwrap = unwrap;
    scope.unwrapIfNeeded = unwrapIfNeeded;
    scope.wrap = wrap;
    scope.wrapIfNeeded = wrapIfNeeded;
    scope.wrappers = wrappers;
  })(window.ShadowDOMPolyfill);
  (function(scope) {
    "use strict";
    function newSplice(index, removed, addedCount) {
      return {
        index: index,
        removed: removed,
        addedCount: addedCount
      };
    }
    var EDIT_LEAVE = 0;
    var EDIT_UPDATE = 1;
    var EDIT_ADD = 2;
    var EDIT_DELETE = 3;
    function ArraySplice() {}