**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

meter: "HTMLMeterElement",
      object: "HTMLObjectElement",
      ol: "HTMLOListElement",
      optgroup: "HTMLOptGroupElement",
      option: "HTMLOptionElement",
      output: "HTMLOutputElement",
      p: "HTMLParagraphElement",
      param: "HTMLParamElement",
      pre: "HTMLPreElement",
      progress: "HTMLProgressElement",
      q: "HTMLQuoteElement",
      script: "HTMLScriptElement",
      select: "HTMLSelectElement",
      shadow: "HTMLShadowElement",
      source: "HTMLSourceElement",
      span: "HTMLSpanElement",
      style: "HTMLStyleElement",
      table: "HTMLTableElement",
      tbody: "HTMLTableSectionElement",
      template: "HTMLTemplateElement",
      textarea: "HTMLTextAreaElement",
      thead: "HTMLTableSectionElement",
      time: "HTMLTimeElement",
      title: "HTMLTitleElement",
      tr: "HTMLTableRowElement",
      track: "HTMLTrackElement",
      ul: "HTMLUListElement",
      video: "HTMLVideoElement"
    };
    function overrideConstructor(tagName) {