**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 (!window[name]) return;
      assert(!scope.wrappers[name]);
      var GeneratedWrapper = function(node) {
        HTMLElement.call(this, node);
      };
      GeneratedWrapper.prototype = Object.create(HTMLElement.prototype);
      mixin(GeneratedWrapper.prototype, {
        get form() {
          return wrap(unwrap(this).form);
        }
      });
      registerWrapper(window[name], GeneratedWrapper, document.createElement(name.slice(4, -7)));
      scope.wrappers[name] = GeneratedWrapper;
    }
    elementsWithFormProperty.forEach(createWrapperConstructor);
  })(window.ShadowDOMPolyfill);
  (function(scope) {
    "use strict";
    var registerWrapper = scope.registerWrapper;
    var setWrapper = scope.setWrapper;
    var unsafeUnwrap = scope.unsafeUnwrap;
    var unwrap = scope.unwrap;
    var unwrapIfNeeded = scope.unwrapIfNeeded;
    var wrap = scope.wrap;
    var OriginalSelection = window.Selection;
    function Selection(impl) {
      setWrapper(impl, this);
    }
    Selection.prototype = {
      get anchorNode() {