**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 used = {};
    var p = inSrc;
    while (p !== inNative && p !== HTMLElement.prototype) {
      var keys = Object.getOwnPropertyNames(p);
      for (var i = 0, k; k = keys[i]; i++) {
        if (!used[k]) {
          Object.defineProperty(inTarget, k, Object.getOwnPropertyDescriptor(p, k));
          used[k] = 1;
        }
      }
      p = Object.getPrototypeOf(p);
    }
  }
  function created(element) {
    if (element.createdCallback) {
      element.createdCallback();
    }
  }
  scope.upgrade = upgrade;
  scope.upgradeWithDefinition = upgradeWithDefinition;
  scope.implementPrototype = implementPrototype;
});
CustomElements.addModule(function(scope) {
  var upgradeDocumentTree = scope.upgradeDocumentTree;
  var upgrade = scope.upgrade;
  var upgradeWithDefinition = scope.upgradeWithDefinition;
  var implementPrototype = scope.implementPrototype;
  var useNative = scope.useNative;
  function register(name, options) {