**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

}
    const newRoot = DOMRenderer.createContainer(container);
    root = container._reactRootContainer = newRoot;
    
    DOMRenderer.unbatchedUpdates(() => {
      DOMRenderer.updateContainer(children, newRoot, parentComponent, callback);
    });
  } else {
    DOMRenderer.updateContainer(children, root, parentComponent, callback);
  }
  return DOMRenderer.getPublicRootInstance(root);
}
function createPortal(
  children: ReactNodeList,
  container: DOMContainer,
  key: ?string = null,
) {
  invariant(
    isValidContainer(container),
    'Target container is not a DOM element.',
  );
  
  return ReactPortal.createPortal(children, container, null, key);
}
var ReactDOMFiber = {
  createPortal,
  findDOMNode(
    componentOrElement: Element | ?React$Component<any, any>,
  ): null | Element | Text {
    if (__DEV__) {