**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 owner = (ReactCurrentOwner.current: any);
      if (owner !== null) {
        var warnedAboutRefsInRender = owner.stateNode._warnedAboutRefsInRender;
        warning(
          warnedAboutRefsInRender,
          '%s is accessing findDOMNode inside its render(). ' +
            'render() should be a pure function of props and state. It should ' +
            'never access something that requires stale data from the previous ' +
            'render, such as refs. Move this logic to componentDidMount and ' +
            'componentDidUpdate instead.',
          getComponentName(owner) || 'A component',
        );
        owner.stateNode._warnedAboutRefsInRender = true;
      }
    }
    if (componentOrElement == null) {
      return null;
    }
    if ((componentOrElement: any).nodeType === ELEMENT_NODE) {
      return (componentOrElement: any);
    }
    var inst = ReactInstanceMap.get(componentOrElement);
    if (inst) {
      return DOMRenderer.findHostInstance(inst);
    }