**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 node instanceof HTMLContentElement || node instanceof HTMLShadowElement;
    }
    function isShadowHost(shadowHost) {
      return shadowHost.shadowRoot;
    }
    function getShadowTrees(host) {
      var trees = [];
      for (var tree = host.shadowRoot; tree; tree = tree.olderShadowRoot) {
        trees.push(tree);
      }
      return trees;
    }
    function render(host) {
      new ShadowRenderer(host).render();
    }
    Node.prototype.invalidateShadowRenderer = function(force) {
      var renderer = unsafeUnwrap(this).polymerShadowRenderer_;
      if (renderer) {
        renderer.invalidate();
        return true;
      }
      return false;
    };
    HTMLContentElement.prototype.getDistributedNodes = HTMLShadowElement.prototype.getDistributedNodes = function() {
      renderAllPending();
      return getDistributedNodes(this);
    };
    Element.prototype.getDestinationInsertionPoints = function() {