**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

return constructorTable.get(nativeConstructor.prototype) === wrapperConstructor;
    }
    function registerObject(object) {
      var nativePrototype = Object.getPrototypeOf(object);
      var superWrapperConstructor = getWrapperConstructor(nativePrototype);
      var GeneratedWrapper = createWrapperConstructor(superWrapperConstructor);
      registerInternal(nativePrototype, GeneratedWrapper, object);
      return GeneratedWrapper;
    }
    function createWrapperConstructor(superWrapperConstructor) {
      function GeneratedWrapper(node) {
        superWrapperConstructor.call(this, node);
      }
      var p = Object.create(superWrapperConstructor.prototype);
      p.constructor = GeneratedWrapper;
      GeneratedWrapper.prototype = p;
      return GeneratedWrapper;
    }
    function isWrapper(object) {
      return object && object.__impl4cf1e782hg__;
    }
    function isNative(object) {
      return !isWrapper(object);
    }
    function wrap(impl) {
      if (impl === null) return null;
      assert(isNative(impl));
      return impl.__wrapper8e3dd93a60__ || (impl.__wrapper8e3dd93a60__ = new (getWrapperConstructor(impl))(impl));
    }
    function unwrap(wrapper) {