**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

}
    } : function(parent, child) {
      originalRemoveChild.call(parent, child);
    };
    Node.prototype = Object.create(EventTarget.prototype);
    mixin(Node.prototype, {
      appendChild: function(childWrapper) {
        return this.insertBefore(childWrapper, null);
      },
      insertBefore: function(childWrapper, refWrapper) {
        assertIsNodeWrapper(childWrapper);
        var refNode;
        if (refWrapper) {
          if (isWrapper(refWrapper)) {
            refNode = unwrap(refWrapper);
          } else {
            refNode = refWrapper;
            refWrapper = wrap(refNode);
          }
        } else {
          refWrapper = null;
          refNode = null;
        }
        refWrapper && assert(refWrapper.parentNode === this);
        var nodes;
        var previousNode = refWrapper ? refWrapper.previousSibling : this.lastChild;
        var useNative = !this.invalidateShadowRenderer() && !invalidateParent(childWrapper);
        if (useNative) nodes = collectNodesNative(childWrapper); else nodes = collectNodes(childWrapper, this, previousNode, refWrapper);
        if (useNative) {
          ensureSameOwnerDocument(this, childWrapper);