**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

clearChildNodes(this);
          originalInsertBefore.call(unsafeUnwrap(this), unwrap(childWrapper), refNode);
        } else {
          if (!previousNode) this.firstChild_ = nodes[0];
          if (!refWrapper) {
            this.lastChild_ = nodes[nodes.length - 1];
            if (this.firstChild_ === undefined) this.firstChild_ = this.firstChild;
          }
          var parentNode = refNode ? refNode.parentNode : unsafeUnwrap(this);
          if (parentNode) {
            originalInsertBefore.call(parentNode, unwrapNodesForInsertion(this, nodes), refNode);
          } else {
            adoptNodesIfNeeded(this, nodes);
          }
        }
        enqueueMutation(this, "childList", {
          addedNodes: nodes,
          nextSibling: refWrapper,
          previousSibling: previousNode
        });
        nodesWereAdded(nodes, this);
        return childWrapper;
      },
      removeChild: function(childWrapper) {
        assertIsNodeWrapper(childWrapper);
        if (childWrapper.parentNode !== this) {
          var found = false;
          var childNodes = this.childNodes;
          for (var ieChild = this.firstChild; ieChild; ieChild = ieChild.nextSibling) {
            if (ieChild === childWrapper) {