**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

flags.log = {};
    }
  }
  flags.shadow = flags.shadow || flags.shadowdom || flags.polyfill;
  if (flags.shadow === "native") {
    flags.shadow = false;
  } else {
    flags.shadow = flags.shadow || !HTMLElement.prototype.createShadowRoot;
  }
  if (flags.register) {
    window.CustomElements = window.CustomElements || {
      flags: {}
    };
    window.CustomElements.flags.register = flags.register;
  }
  scope.flags = flags;
})(WebComponents);
if (WebComponents.flags.shadow) {
  if (typeof WeakMap === "undefined") {
    (function() {
      var defineProperty = Object.defineProperty;
      var counter = Date.now() % 1e9;
      var WeakMap = function() {
        this.name = "__st" + (Math.random() * 1e9 >>> 0) + (counter++ + "__");
      };
      WeakMap.prototype = {
        set: function(key, value) {
          var entry = key[this.name];
          if (entry && entry[0] === key) entry[1] = value; else defineProperty(key, this.name, {
            value: [ key, value ],