**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

definition.__name = name.toLowerCase();
    definition.lifecycle = definition.lifecycle || {};
    definition.ancestry = ancestry(definition.extends);
    resolveTagName(definition);
    resolvePrototypeChain(definition);
    overrideAttributeApi(definition.prototype);
    registerDefinition(definition.__name, definition);
    definition.ctor = generateConstructor(definition);
    definition.ctor.prototype = definition.prototype;
    definition.prototype.constructor = definition.ctor;
    if (scope.ready) {
      upgradeDocumentTree(document);
    }
    return definition.ctor;
  }
  function overrideAttributeApi(prototype) {
    if (prototype.setAttribute._polyfilled) {
      return;
    }
    var setAttribute = prototype.setAttribute;
    prototype.setAttribute = function(name, value) {
      changeAttribute.call(this, name, value, setAttribute);
    };
    var removeAttribute = prototype.removeAttribute;
    prototype.removeAttribute = function(name) {
      changeAttribute.call(this, name, null, removeAttribute);
    };
    prototype.setAttribute._polyfilled = true;
  }
  function changeAttribute(name, value, operation) {