**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

pendingMutations.push(fn);
    if (!isPendingMutations) {
      isPendingMutations = true;
      setTimeout(takeMutations);
    }
  }
  function takeMutations() {
    isPendingMutations = false;
    var $p = pendingMutations;
    for (var i = 0, l = $p.length, p; i < l && (p = $p[i]); i++) {
      p();
    }
    pendingMutations = [];
  }
  function attached(element) {
    if (hasPolyfillMutations) {
      deferMutation(function() {
        _attached(element);
      });
    } else {
      _attached(element);
    }
  }
  function _attached(element) {