**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

for (var i = 0, l = nodes.length, n, loading; i < l && (n = nodes[i]); i++) {
        if (n.children && n.children.length) {
          this.addedNodes(n.children);
        }
      }
    },
    observe: function(root) {
      this.mo.observe(root, {
        childList: true,
        subtree: true
      });
    }
  };
  scope.Observer = Observer;
});
HTMLImports.addModule(function(scope) {
  var path = scope.path;
  var rootDocument = scope.rootDocument;
  var flags = scope.flags;
  var isIE = scope.isIE;
  var IMPORT_LINK_TYPE = scope.IMPORT_LINK_TYPE;
  var IMPORT_SELECTOR = "link[rel=" + IMPORT_LINK_TYPE + "]";
  var importParser = {
    documentSelectors: IMPORT_SELECTOR,
    importsSelectors: [ IMPORT_SELECTOR, "link[rel=stylesheet]", "style", "script:not([type])", 'script[type="text/javascript"]' ].join(","),
    map: {
      link: "parseLink",
      script: "parseScript",
      style: "parseStyle"
    },