**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 nativePrototype;
        var prototypes = [];
        while (proto) {
          nativePrototype = scope.nativePrototypeTable.get(proto);
          if (nativePrototype) break;
          prototypes.push(proto);
          proto = Object.getPrototypeOf(proto);
        }
        if (!nativePrototype) {
          throw new Error("NotSupportedError");
        }
        var newPrototype = Object.create(nativePrototype);
        for (var i = prototypes.length - 1; i >= 0; i--) {
          newPrototype = Object.create(newPrototype);
        }
        [ "createdCallback", "attachedCallback", "detachedCallback", "attributeChangedCallback" ].forEach(function(name) {
          var f = prototype[name];
          if (!f) return;
          newPrototype[name] = function() {
            if (!(wrap(this) instanceof CustomElementConstructor)) {
              rewrap(this);
            }
            f.apply(wrap(this), arguments);
          };
        });
        var p = {
          prototype: newPrototype
        };
        if (extendsOption) p.extends = extendsOption;
        function CustomElementConstructor(node) {