**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

name = name.toLowerCase();
    var oldValue = this.getAttribute(name);
    operation.apply(this, arguments);
    var newValue = this.getAttribute(name);
    if (this.attributeChangedCallback && newValue !== oldValue) {
      this.attributeChangedCallback(name, oldValue, newValue);
    }
  }
  function isReservedTag(name) {
    for (var i = 0; i < reservedTagList.length; i++) {
      if (name === reservedTagList[i]) {
        return true;
      }
    }
  }
  var reservedTagList = [ "annotation-xml", "color-profile", "font-face", "font-face-src", "font-face-uri", "font-face-format", "font-face-name", "missing-glyph" ];
  function ancestry(extnds) {
    var extendee = getRegisteredDefinition(extnds);
    if (extendee) {
      return ancestry(extendee.extends).concat([ extendee ]);
    }
    return [];
  }
  function resolveTagName(definition) {