**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

var newIndex = 0, oldIndex = 0;
        var lastIndex = 0;
        for (var i = 0; i < splices.length; i++) {
          var splice = splices[i];
          for (;lastIndex < splice.index; lastIndex++) {
            oldIndex++;
            newChildren[newIndex++].sync(added);
          }
          var removedCount = splice.removed.length;
          for (var j = 0; j < removedCount; j++) {
            var wrapper = wrap(oldChildren[oldIndex++]);
            if (!added.get(wrapper)) remove(wrapper);
          }
          var addedCount = splice.addedCount;
          var refNode = oldChildren[oldIndex] && wrap(oldChildren[oldIndex]);
          for (var j = 0; j < addedCount; j++) {
            var newChildRenderNode = newChildren[newIndex++];
            var newChildWrapper = newChildRenderNode.node;
            insertBefore(nodeWrapper, newChildWrapper, refNode);
            added.set(newChildWrapper, true);
            newChildRenderNode.sync(added);
          }
          lastIndex += addedCount;
        }
        for (var i = lastIndex; i < newChildren.length; i++) {
          newChildren[i].sync(added);
        }
      }
    };
    function ShadowRenderer(host) {