**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

}
        return splices;
      },
      sharedPrefix: function(current, old, searchLength) {
        for (var i = 0; i < searchLength; i++) if (!this.equals(current[i], old[i])) return i;
        return searchLength;
      },
      sharedSuffix: function(current, old, searchLength) {
        var index1 = current.length;
        var index2 = old.length;
        var count = 0;
        while (count < searchLength && this.equals(current[--index1], old[--index2])) count++;
        return count;
      },
      calculateSplices: function(current, previous) {
        return this.calcSplices(current, 0, current.length, previous, 0, previous.length);
      },
      equals: function(currentValue, previousValue) {
        return currentValue === previousValue;
      }
    };
    scope.ArraySplice = ArraySplice;
  })(window.ShadowDOMPolyfill);
  (function(context) {
    "use strict";
    var OriginalMutationObserver = window.MutationObserver;
    var callbacks = [];
    var pending = false;
    var timerFunc;
    function handle() {