**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

autoFocus?: boolean,
  children?: mixed,
  hidden?: boolean,
};
type Instance = Element;
type TextInstance = Text;
type HostContextDev = {
  namespace: string,
  ancestorInfo: mixed,
};
type HostContextProd = string;
type HostContext = HostContextDev | HostContextProd;
let eventsEnabled: ?boolean = null;
let selectionInformation: ?mixed = null;
/**
 * True if the supplied DOM node is a valid node element.
 *
 * @param {?DOMElement} node The candidate DOM node.
 * @return {boolean} True if the DOM is a valid DOM node.
 * @internal
 */
function isValidContainer(node) {
  return !!(node &&
    (node.nodeType === ELEMENT_NODE ||
      node.nodeType === DOCUMENT_NODE ||
      node.nodeType === DOCUMENT_FRAGMENT_NODE ||
      (node.nodeType === COMMENT_NODE &&
        node.nodeValue === ' react-mount-point-unstable ')));
}
function getReactRootElementInContainer(container: any) {