**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

name: name,
          extendsName: extendsName
        };
        var styles = this.findStyles(root);
        def.rootStyles = styles;
        def.scopeStyles = def.rootStyles;
        var extendee = this.registry[def.extendsName];
        if (extendee) {
          def.scopeStyles = extendee.scopeStyles.concat(def.scopeStyles);
        }
        return def;
      },
      findStyles: function(root) {
        if (!root) {
          return [];
        }
        var styles = root.querySelectorAll("style");
        return Array.prototype.filter.call(styles, function(s) {
          return !s.hasAttribute(NO_SHIM_ATTRIBUTE);
        });
      },
      applyScopeToContent: function(root, name) {
        if (root) {
          Array.prototype.forEach.call(root.querySelectorAll("*"), function(node) {
            node.setAttribute(name, "");
          });
          Array.prototype.forEach.call(root.querySelectorAll("template"), function(template) {
            this.applyScopeToContent(template.content, name);
          }, this);
        }