**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 x >>> 0;
    }
    var OriginalText = window.Text;
    function Text(node) {
      CharacterData.call(this, node);
    }
    Text.prototype = Object.create(CharacterData.prototype);
    mixin(Text.prototype, {
      splitText: function(offset) {
        offset = toUInt32(offset);
        var s = this.data;
        if (offset > s.length) throw new Error("IndexSizeError");
        var head = s.slice(0, offset);
        var tail = s.slice(offset);
        this.data = head;
        var newTextNode = this.ownerDocument.createTextNode(tail);
        if (this.parentNode) this.parentNode.insertBefore(newTextNode, this.nextSibling);
        return newTextNode;
      }
    });
    registerWrapper(OriginalText, Text, document.createTextNode(""));
    scope.wrappers.Text = Text;
  })(window.ShadowDOMPolyfill);
  (function(scope) {
    "use strict";
    var setWrapper = scope.setWrapper;
    var unsafeUnwrap = scope.unsafeUnwrap;
    function invalidateClass(el) {