**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

}
    const isRootRenderedBySomeReact = !!container._reactRootContainer;
    const rootEl = getReactRootElementInContainer(container);
    const hasNonRootReactChild = !!(rootEl &&
      ReactDOMComponentTree.getInstanceFromNode(rootEl));
    warning(
      !hasNonRootReactChild || isRootRenderedBySomeReact,
      'render(...): Replacing React-rendered children with a new root ' +
        'component. If you intended to update the children of this node, ' +
        'you should instead have the existing children update their state ' +
        'and render the new components instead of calling ReactDOM.render.',
    );
    warning(
      container.nodeType !== ELEMENT_NODE ||
        !((container: any): Element).tagName ||
        ((container: any): Element).tagName.toUpperCase() !== 'BODY',
      'render(): Rendering components directly into document.body is ' +
        'discouraged, since its children are often manipulated by third-party ' +
        'scripts and browser extensions. This may lead to subtle ' +
        'reconciliation issues. Try rendering into a container element created ' +
        'for your app.',
    );
  }
  let root = container._reactRootContainer;
  if (!root) {
    const shouldHydrate =
      forceHydrate || shouldHydrateDueToLegacyHeuristic(container);
    
    if (!shouldHydrate) {
      let warned = false;