**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 (flags.dom) {
      var mx = mutations[0];
      if (mx && mx.type === "childList" && mx.addedNodes) {
        if (mx.addedNodes) {
          var d = mx.addedNodes[0];
          while (d && d !== document && !d.host) {
            d = d.parentNode;
          }
          var u = d && (d.URL || d._URL || d.host && d.host.localName) || "";
          u = u.split("/?").shift().split("/").pop();
        }
      }
      console.group("mutations (%d) [%s]", mutations.length, u || "");
    }
    mutations.forEach(function(mx) {
      if (mx.type === "childList") {
        forEach(mx.addedNodes, function(n) {
          if (!n.localName) {
            return;
          }
          addedNode(n);
        });
        forEach(mx.removedNodes, function(n) {
          if (!n.localName) {
            return;
          }
          detachedNode(n);
        });
      }