**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

this.childList = !!options.childList;
      this.subtree = !!options.subtree;
      if (!("attributes" in options) && ("attributeOldValue" in options || "attributeFilter" in options)) {
        this.attributes = true;
      } else {
        this.attributes = !!options.attributes;
      }
      if ("characterDataOldValue" in options && !("characterData" in options)) this.characterData = true; else this.characterData = !!options.characterData;
      if (!this.attributes && (options.attributeOldValue || "attributeFilter" in options) || !this.characterData && options.characterDataOldValue) {
        throw new TypeError();
      }
      this.characterData = !!options.characterData;
      this.attributeOldValue = !!options.attributeOldValue;
      this.characterDataOldValue = !!options.characterDataOldValue;
      if ("attributeFilter" in options) {
        if (options.attributeFilter == null || typeof options.attributeFilter !== "object") {
          throw new TypeError();
        }
        this.attributeFilter = slice.call(options.attributeFilter);
      } else {
        this.attributeFilter = null;
      }
    }
    var uidCounter = 0;
    function MutationObserver(callback) {