**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

var target = unsafeUnwrap(this);
      var list;
      var root = getTreeScope(this).root;
      if (root instanceof scope.wrappers.ShadowRoot) {
        return findElements(this, index, result, p, ns, localName);
      } else if (target instanceof OriginalElement) {
        list = originalElementGetElementsByTagNameNS.call(target, ns, localName);
      } else if (target instanceof OriginalDocument) {
        list = originalDocumentGetElementsByTagNameNS.call(target, ns, localName);
      } else {
        return findElements(this, index, result, p, ns, localName);
      }
      return filterNodeList(list, index, result, false);
    }
    var GetElementsByInterface = {
      getElementsByTagName: function(localName) {
        var result = new HTMLCollection();
        var match = localName === "*" ? matchesEveryThing : matchesTagName;
        result.length = getElementsByTagNameFiltered.call(this, match, 0, result, localName, localName.toLowerCase());
        return result;
      },
      getElementsByClassName: function(className) {
        return this.querySelectorAll("." + className);
      },
      getElementsByTagNameNS: function(ns, localName) {
        var result = new HTMLCollection();