**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

render() {
        return <div><div /></div>;
      }
    }
    ReactDOM.render(<Component />, container);
    // 
    spyOn(console, 'error');
    var rootNode = container.firstChild;
    ReactDOM.render(<span />, rootNode);
    expectDev(console.error.calls.count()).toBe(1);
    expectDev(console.error.calls.argsFor(0)[0]).toBe(
      'Warning: render(...): Replacing React-rendered children with a new ' +
        'root component. If you intended to update the children of this node, ' +
        'you should instead have the existing children update their state and ' +
        'render the new components instead of calling ReactDOM.render.',
    );
  });
  it('should warn if the unmounted node was rendered by another copy of React', () => {
    jest.resetModules();
    var ReactDOMOther = require('react-dom');
    var container = document.createElement('div');
    class Component extends React.Component {