**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 (type === "attributes" && options.attributeFilter && (data.namespace !== null || options.attributeFilter.indexOf(data.name) === -1)) {
            continue;
          }
          if (type === "characterData" && !options.characterData) continue;
          if (type === "childList" && !options.childList) continue;
          var observer = registration.observer;
          interestedObservers[observer.uid_] = observer;
          if (type === "attributes" && options.attributeOldValue || type === "characterData" && options.characterDataOldValue) {
            associatedStrings[observer.uid_] = data.oldValue;
          }
        }
      }
      for (var uid in interestedObservers) {
        var observer = interestedObservers[uid];
        var record = new MutationRecord(type, target);
        if ("name" in data && "namespace" in data) {
          record.attributeName = data.name;
          record.attributeNamespace = data.namespace;
        }
        if (data.addedNodes) record.addedNodes = data.addedNodes;
        if (data.removedNodes) record.removedNodes = data.removedNodes;
        if (data.previousSibling) record.previousSibling = data.previousSibling;
        if (data.nextSibling) record.nextSibling = data.nextSibling;
        if (associatedStrings[uid] !== undefined) record.oldValue = associatedStrings[uid];
        scheduleCallback(observer);
        observer.records_.push(record);
      }
    }
    var slice = Array.prototype.slice;
    function MutationObserverOptions(options) {