**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 (options.attributes) node.addEventListener("DOMAttrModified", this, true);
      if (options.characterData) node.addEventListener("DOMCharacterDataModified", this, true);
      if (options.childList) node.addEventListener("DOMNodeInserted", this, true);
      if (options.childList || options.subtree) node.addEventListener("DOMNodeRemoved", this, true);
    },
    removeListeners: function() {
      this.removeListeners_(this.target);
    },
    removeListeners_: function(node) {
      var options = this.options;
      if (options.attributes) node.removeEventListener("DOMAttrModified", this, true);
      if (options.characterData) node.removeEventListener("DOMCharacterDataModified", this, true);
      if (options.childList) node.removeEventListener("DOMNodeInserted", this, true);
      if (options.childList || options.subtree) node.removeEventListener("DOMNodeRemoved", this, true);
    },
    addTransientObserver: function(node) {
      if (node === this.target) return;
      this.addListeners_(node);
      this.transientObservedNodes.push(node);
      var registrations = registrationsTable.get(node);
      if (!registrations) registrationsTable.set(node, registrations = []);
      registrations.push(this);
    },
    removeTransientObservers: function() {
      var transientObservedNodes = this.transientObservedNodes;
      this.transientObservedNodes = [];
      transientObservedNodes.forEach(function(node) {
        this.removeListeners_(node);
        var registrations = registrationsTable.get(node);
        for (var i = 0; i < registrations.length; i++) {