**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

nextSibling: childWrapperNextSibling,
            previousSibling: childWrapperPreviousSibling
          });
        }
        registerTransientObservers(this, childWrapper);
        return childWrapper;
      },
      replaceChild: function(newChildWrapper, oldChildWrapper) {
        assertIsNodeWrapper(newChildWrapper);
        var oldChildNode;
        if (isWrapper(oldChildWrapper)) {
          oldChildNode = unwrap(oldChildWrapper);
        } else {
          oldChildNode = oldChildWrapper;
          oldChildWrapper = wrap(oldChildNode);
        }
        if (oldChildWrapper.parentNode !== this) {
          throw new Error("NotFoundError");
        }
        var nextNode = oldChildWrapper.nextSibling;
        var previousNode = oldChildWrapper.previousSibling;
        var nodes;
        var useNative = !this.invalidateShadowRenderer() && !invalidateParent(newChildWrapper);
        if (useNative) {
          nodes = collectNodesNative(newChildWrapper);
        } else {
          if (nextNode === newChildWrapper) nextNode = newChildWrapper.nextSibling;
          nodes = collectNodes(newChildWrapper, this, previousNode, nextNode);
        }
        if (!useNative) {