**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

writable: true
          });
          return this;
        },
        get: function(key) {
          var entry;
          return (entry = key[this.name]) && entry[0] === key ? entry[1] : undefined;
        },
        "delete": function(key) {
          var entry = key[this.name];
          if (!entry || entry[0] !== key) return false;
          entry[0] = entry[1] = undefined;
          return true;
        },
        has: function(key) {
          var entry = key[this.name];
          if (!entry) return false;
          return entry[0] === key;
        }
      };
      window.WeakMap = WeakMap;
    })();
  }
  window.ShadowDOMPolyfill = {};
  (function(scope) {
    "use strict";
    var constructorTable = new WeakMap();
    var nativePrototypeTable = new WeakMap();
    var wrappers = Object.create(null);
    function detectEval() {