**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 (!document.baseURI) {
    var baseURIDescriptor = {
      get: function() {
        var base = document.querySelector("base");
        return base ? base.href : window.location.href;
      },
      configurable: true
    };
    Object.defineProperty(document, "baseURI", baseURIDescriptor);
    Object.defineProperty(rootDocument, "baseURI", baseURIDescriptor);
  }
  scope.importer = importer;
  scope.importLoader = importLoader;
});
HTMLImports.addModule(function(scope) {
  var parser = scope.parser;
  var importer = scope.importer;
  var dynamic = {
    added: function(nodes) {
      var owner, parsed;
      for (var i = 0, l = nodes.length, n; i < l && (n = nodes[i]); i++) {
        if (!owner) {
          owner = n.ownerDocument;
          parsed = parser.isParsed(owner);
        }
        loading = this.shouldLoadNode(n);
        if (loading) {
          importer.loadNode(n);
        }