**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 wrap(this || window).getDefaultComputedStyle(unwrapIfNeeded(el), pseudo);
      };
    }
    OriginalWindow.prototype.getSelection = function() {
      return wrap(this || window).getSelection();
    };
    delete window.getComputedStyle;
    delete window.getDefaultComputedStyle;
    delete window.getSelection;
    [ "addEventListener", "removeEventListener", "dispatchEvent" ].forEach(function(name) {
      OriginalWindow.prototype[name] = function() {
        var w = wrap(this || window);
        return w[name].apply(w, arguments);
      };
      delete window[name];
    });
    mixin(Window.prototype, {
      getComputedStyle: function(el, pseudo) {
        renderAllPending();
        return originalGetComputedStyle.call(unwrap(this), unwrapIfNeeded(el), pseudo);
      },
      getSelection: function() {
        renderAllPending();
        return new Selection(originalGetSelection.call(unwrap(this)));
      },
      get document() {
        return wrap(unwrap(this).document);
      }
    });
    if (originalGetDefaultComputedStyle) {