**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

callback(elementDeclarations);
    }
    elementDeclarations = null;
  };
  function installPolymerWarning() {
    if (window.Polymer === polymerStub) {
      window.Polymer = function() {
        throw new Error("You tried to use polymer without loading it first. To " + 'load polymer, <link rel="import" href="' + 'components/polymer/polymer.html">');
      };
    }
  }
  if (HTMLImports.useNative) {
    installPolymerWarning();
  } else {
    addEventListener("DOMContentLoaded", installPolymerWarning);
  }
})(window.WebComponents);
(function(scope) {
  var style = document.createElement("style");
  style.textContent = "" + "body {" + "transition: opacity ease-in 0.2s;" + " } \n" + "body[unresolved] {" + "opacity: 0; display: block; overflow: hidden; position: relative;" + " } \n";
  var head = document.querySelector("head");
  head.insertBefore(style, head.firstChild);
})(window.WebComponents);
(function(scope) {
  window.Platform = scope;
})(window.WebComponents);
if (typeof exports !== 'undefined') {
  module.exports = window.WebComponents;
}