**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

upgradeDocumentTree(wrap(document));
    if (window.HTMLImports) {
      HTMLImports.__importsParsingHook = function(elt) {
        upgradeDocumentTree(wrap(elt.import));
      };
    }
    CustomElements.ready = true;
    setTimeout(function() {
      CustomElements.readyTime = Date.now();
      if (window.HTMLImports) {
        CustomElements.elapsed = CustomElements.readyTime - HTMLImports.readyTime;
      }
      document.dispatchEvent(new CustomEvent("WebComponentsReady", {
        bubbles: true
      }));
    });
  }
  if (typeof window.CustomEvent !== "function") {
    window.CustomEvent = function(inType, params) {
      params = params || {};
      var e = document.createEvent("CustomEvent");
      e.initCustomEvent(inType, Boolean(params.bubbles), Boolean(params.cancelable), params.detail);
      return e;
    };
    window.CustomEvent.prototype = window.Event.prototype;
  }
  if (document.readyState === "complete" || scope.flags.eager) {
    bootstrap();
  } else if (document.readyState === "interactive" && !window.attachEvent && (!window.HTMLImports || window.HTMLImports.ready)) {
    bootstrap();