**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

expectDev(console.error.calls.argsFor(0)[0]).toContain(
      'Server: "This markup contains an nbsp entity:   server text" ' +
        'Client: "This markup contains an nbsp entity:   client text"',
    );
  });
  if (WebComponents !== undefined) {
    it('should allow mounting/unmounting to document fragment container', () => {
      var shadowRoot;
      var proto = Object.create(HTMLElement.prototype, {
        createdCallback: {
          value: function() {
            shadowRoot = this.createShadowRoot();
            ReactDOM.render(<div>Hi, from within a WC!</div>, shadowRoot);
            expect(shadowRoot.firstChild.tagName).toBe('DIV');
            ReactDOM.render(<span>Hi, from within a WC!</span>, shadowRoot);
            expect(shadowRoot.firstChild.tagName).toBe('SPAN');
          },
        },
      });
      proto.unmount = function() {
        ReactDOM.unmountComponentAtNode(shadowRoot);
      };
      document.registerElement('x-foo', {prototype: proto});
      var element = document.createElement('x-foo');
      element.unmount();
    });
  }
  it('should warn if render removes React-rendered children', () => {
    var container = document.createElement('container');
    class Component extends React.Component {