**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 original = document[name];
      Document.prototype[name] = function() {
        return wrap(original.apply(unsafeUnwrap(this), arguments));
      };
    }
    [ "createComment", "createDocumentFragment", "createElement", "createElementNS", "createEvent", "createEventNS", "createRange", "createTextNode", "getElementById" ].forEach(wrapMethod);
    var originalAdoptNode = document.adoptNode;
    function adoptNodeNoRemove(node, doc) {
      originalAdoptNode.call(unsafeUnwrap(doc), unwrap(node));
      adoptSubtree(node, doc);
    }
    function adoptSubtree(node, doc) {
      if (node.shadowRoot) doc.adoptNode(node.shadowRoot);
      if (node instanceof ShadowRoot) adoptOlderShadowRoots(node, doc);
      for (var child = node.firstChild; child; child = child.nextSibling) {
        adoptSubtree(child, doc);
      }
    }
    function adoptOlderShadowRoots(shadowRoot, doc) {
      var oldShadowRoot = shadowRoot.olderShadowRoot;
      if (oldShadowRoot) doc.adoptNode(oldShadowRoot);
    }
    var originalGetSelection = document.getSelection;
    mixin(Document.prototype, {
      adoptNode: function(node) {
        if (node.parentNode) node.parentNode.removeChild(node);