**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

if (element.__upgraded__ && (element.attachedCallback || element.detachedCallback)) {
      if (element.__attached && !inDocument(element)) {
        element.__attached = false;
        if (element.detachedCallback) {
          element.detachedCallback();
        }
      }
    }
  }
  function inDocument(element) {
    var p = element;
    var doc = wrap(document);
    while (p) {
      if (p == doc) {
        return true;
      }
      p = p.parentNode || p.host;
    }
  }
  function watchShadow(node) {
    if (node.shadowRoot && !node.shadowRoot.__watched) {
      flags.dom && console.log("watching shadow-root for: ", node.localName);
      var root = node.shadowRoot;
      while (root) {
        observe(root);
        root = root.olderShadowRoot;
      }
    }
  }
  function handler(mutations) {