**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

getDistributedNodes(insertionPoint).push(child);
      var points = destinationInsertionPointsTable.get(child);
      if (!points) destinationInsertionPointsTable.set(child, [ insertionPoint ]); else points.push(insertionPoint);
    }
    function getDestinationInsertionPoints(node) {
      return destinationInsertionPointsTable.get(node);
    }
    function resetDestinationInsertionPoints(node) {
      destinationInsertionPointsTable.set(node, undefined);
    }
    var selectorStartCharRe = /^(:not\()?[*.#[a-zA-Z_|]/;
    function matches(node, contentElement) {
      var select = contentElement.getAttribute("select");
      if (!select) return true;
      select = select.trim();
      if (!select) return true;
      if (!(node instanceof Element)) return false;
      if (!selectorStartCharRe.test(select)) return false;
      try {
        return node.matches(select);
      } catch (ex) {
        return false;
      }
    }
    function isFinalDestination(insertionPoint, node) {
      var points = getDestinationInsertionPoints(node);
      return points && points[points.length - 1] === insertionPoint;
    }
    function isInsertionPoint(node) {