**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

window.location.pathname = event.target.value;
  }
  render() {
    return (
      <header className="header">
        <div className="header__inner">
          <span className="header__logo">
            <img
              src="https://reactjs.org/img/logo.svg"
              alt=""
              width="32"
              height="32"
            />
            React Sandbox (v{React.version})
          </span>
          <div className="header-controls">
            <label htmlFor="example">
              <span className="sr-only">Select an example</span>
              <select
                value={window.location.pathname}
                onChange={this.handleFixtureChange}>
                <option value="/">Select a Fixture</option>
                <option value="/range-inputs">Range Inputs</option>
                <option value="/text-inputs">Text Inputs</option>
                <option value="/number-inputs">Number Input</option>