**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

if (!ts.parent) return ts;
      return getTreeScopeRoot(ts.parent);
    }
    function relatedTargetResolution(event, currentTarget, relatedTarget) {
      if (currentTarget instanceof wrappers.Window) currentTarget = currentTarget.document;
      var currentTargetTree = getTreeScope(currentTarget);
      var relatedTargetTree = getTreeScope(relatedTarget);
      var relatedTargetEventPath = getEventPath(relatedTarget, event);
      var lowestCommonAncestorTree;
      var lowestCommonAncestorTree = lowestCommonInclusiveAncestor(currentTargetTree, relatedTargetTree);
      if (!lowestCommonAncestorTree) lowestCommonAncestorTree = relatedTargetTree.root;
      for (var commonAncestorTree = lowestCommonAncestorTree; commonAncestorTree; commonAncestorTree = commonAncestorTree.parent) {
        var adjustedRelatedTarget;
        for (var i = 0; i < relatedTargetEventPath.length; i++) {
          var node = relatedTargetEventPath[i];
          if (getTreeScope(node) === commonAncestorTree) return node;
        }
      }
      return null;
    }
    function inSameTree(a, b) {
      return getTreeScope(a) === getTreeScope(b);
    }
    var NONE = 0;
    var CAPTURING_PHASE = 1;
    var AT_TARGET = 2;
    var BUBBLING_PHASE = 3;
    var pendingError;
    function dispatchOriginalEvent(originalEvent) {