**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

adoptNodeNoRemove(node, this);
        return node;
      },
      elementFromPoint: function(x, y) {
        return elementFromPoint(this, this, x, y);
      },
      importNode: function(node, deep) {
        return cloneNode(node, deep, unsafeUnwrap(this));
      },
      getSelection: function() {
        renderAllPending();
        return new Selection(originalGetSelection.call(unwrap(this)));
      },
      getElementsByName: function(name) {
        return SelectorsInterface.querySelectorAll.call(this, "[name=" + JSON.stringify(String(name)) + "]");
      }
    });
    if (document.registerElement) {
      var originalRegisterElement = document.registerElement;
      Document.prototype.registerElement = function(tagName, object) {
        var prototype, extendsOption;
        if (object !== undefined) {
          prototype = object.prototype;
          extendsOption = object.extends;
        }
        if (!prototype) prototype = Object.create(HTMLElement.prototype);
        if (scope.nativePrototypeTable.get(prototype)) {
          throw new Error("NotSupportedError");
        }
        var proto = Object.getPrototypeOf(prototype);