**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

<option value="/password-inputs">Password Input</option>
                <option value="/selects">Selects</option>
                <option value="/textareas">Textareas</option>
                <option value="/input-change-events">
                  Input change events
                </option>
                <option value="/buttons">Buttons</option>
                <option value="/date-inputs">Date Inputs</option>
                <option value="/error-handling">Error Handling</option>
                <option value="/event-pooling">Event Pooling</option>
              </select>
            </label>
            <label htmlFor="react_version">
              <span className="sr-only">Select a version to test</span>
              <select
                value={this.state.version}
                onChange={this.handleVersionChange}>
                {this.state.versions.map(version => (
                  <option key={version} value={version}>{version}</option>
                ))}
              </select>
            </label>
          </div>
        </div>
      </header>
    );
  }
}
export default Header;