**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

content: "HTMLContentElement",
      data: "HTMLDataElement",
      datalist: "HTMLDataListElement",
      del: "HTMLModElement",
      dir: "HTMLDirectoryElement",
      div: "HTMLDivElement",
      dl: "HTMLDListElement",
      embed: "HTMLEmbedElement",
      fieldset: "HTMLFieldSetElement",
      font: "HTMLFontElement",
      form: "HTMLFormElement",
      frame: "HTMLFrameElement",
      frameset: "HTMLFrameSetElement",
      h1: "HTMLHeadingElement",
      head: "HTMLHeadElement",
      hr: "HTMLHRElement",
      html: "HTMLHtmlElement",
      iframe: "HTMLIFrameElement",
      img: "HTMLImageElement",
      input: "HTMLInputElement",
      keygen: "HTMLKeygenElement",
      label: "HTMLLabelElement",
      legend: "HTMLLegendElement",
      li: "HTMLLIElement",
      link: "HTMLLinkElement",
      map: "HTMLMapElement",
      marquee: "HTMLMarqueeElement",
      menu: "HTMLMenuElement",
      menuitem: "HTMLMenuItemElement",
      meta: "HTMLMetaElement",