**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

Element.call(this, node);
    }
    HTMLElement.prototype = Object.create(Element.prototype);
    mixin(HTMLElement.prototype, {
      get innerHTML() {
        return getInnerHTML(this);
      },
      set innerHTML(value) {
        if (oldIe && plaintextParents[this.localName]) {
          this.textContent = value;
          return;
        }
        var removedNodes = snapshotNodeList(this.childNodes);
        if (this.invalidateShadowRenderer()) {
          if (this instanceof wrappers.HTMLTemplateElement) setInnerHTML(this.content, value); else setInnerHTML(this, value, this.tagName);
        } else if (!OriginalHTMLTemplateElement && this instanceof wrappers.HTMLTemplateElement) {
          setInnerHTML(this.content, value);
        } else {
          unsafeUnwrap(this).innerHTML = value;
        }
        var addedNodes = snapshotNodeList(this.childNodes);
        enqueueMutation(this, "childList", {
          addedNodes: addedNodes,
          removedNodes: removedNodes
        });
        nodesWereRemoved(removedNodes);
        nodesWereAdded(addedNodes, this);
      },
      get outerHTML() {