**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

var nativeConstructorName = elements[tagName];
      var nativeConstructor = window[nativeConstructorName];
      if (!nativeConstructor) return;
      var element = document.createElement(tagName);
      var wrapperConstructor = element.constructor;
      window[nativeConstructorName] = wrapperConstructor;
    }
    Object.keys(elements).forEach(overrideConstructor);
    Object.getOwnPropertyNames(scope.wrappers).forEach(function(name) {
      window[name] = scope.wrappers[name];
    });
  })(window.ShadowDOMPolyfill);
  (function(scope) {
    var ShadowCSS = {
      strictStyling: false,
      registry: {},
      shimStyling: function(root, name, extendsName) {
        var scopeStyles = this.prepareRoot(root, name, extendsName);
        var typeExtension = this.isTypeExtension(extendsName);
        var scopeSelector = this.makeScopeSelector(name, typeExtension);
        var cssText = stylesToCssText(scopeStyles, true);
        cssText = this.scopeCssText(cssText, scopeSelector);
        if (root) {
          root.shimmedStyle = cssText;
        }
        this.addCssToDocument(cssText, name);
      },
      shimStyle: function(style, selector) {
        return this.shimCssText(style.textContent, selector);
      },