**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

if (!container) {
    return null;
  }
  if (container.nodeType === DOCUMENT_NODE) {
    return container.documentElement;
  } else {
    return container.firstChild;
  }
}
function shouldHydrateDueToLegacyHeuristic(container) {
  const rootElement = getReactRootElementInContainer(container);
  return !!(rootElement &&
    rootElement.nodeType === ELEMENT_NODE &&
    rootElement.hasAttribute(ROOT_ATTRIBUTE_NAME));
}
function shouldAutoFocusHostComponent(type: string, props: Props): boolean {
  switch (type) {
    case 'button':
    case 'input':
    case 'select':
    case 'textarea':
      return !!props.autoFocus;
  }
  return false;
}
var DOMRenderer = ReactFiberReconciler({
  getRootHostContext(rootContainerInstance: Container): HostContext {
    let type;
    let namespace;