We're switching Atom to React, but we want to support backward-compatible shims to our previous view framework.

In the old framework, views were represented as jQuery-wrapped DOM elements. Ideally, we'd like to provide backward-compatible shims by simply rendering the React component and wrapping its root element in a jQuery wrapper with an API that's compatible to our old editor views. That would essentially look like this:

``` javascript
var editorRoot = document.createElement('div')
React.renderComponentWithRootElement(Editor(props), editorRoot)

// the element would actually be wrapped in a custom subclass of jQuery,
// but this is clearer for this example
var editorShim = $(editorRoot) 
```

Currently, this isn't possible because React components need to be rendered into a container. So instead we need to create a wrapper element and render the component inside it. This is sad because it adds an extra layer of markup which confuses styling, and also means that the editor has a wrapper when used via the shim layer but no wrapper when embedded as a child via React itself.

Is there something fundamental preventing React from adopting DOM nodes directly without the parent? Is this something we could even hack in in a way that would be somewhat maintainable?

:heart:

/**
   * Mounted components have a DOM node representation and are capable of
   * receiving new props.
   */
  | 'MOUNTED'
  /**
   * Unmounted components are inactive and cannot receive new props.
   */
  | 'UNMOUNTED';
function getLifeCycleState(instance): ComponentLifeCycle {
  return instance.updater.isMounted(instance) ? 'MOUNTED' : 'UNMOUNTED';
}
/**
 * TODO: We should make any setState calls fail in
 * `getInitialState` and `componentWillMount`. They will usually fail
 * anyways because `this._renderedComponent` is empty, however, if a component
 * is *reused*, then that won't be the case and things will appear to work in
 * some cases. Better to just block all updates in initialization.
 */
describe('ReactComponentLifeCycle', () => {
  beforeEach(() => {
    jest.resetModules();
    React = require('react');
    ReactDOM = require('react-dom');
    ReactTestUtils = require('react-dom/test-utils');
    PropTypes = require('prop-types');
  });
  it('should not reuse an instance when it has been unmounted', () => {
    const container = document.createElement('div');
    class StatefulComponent extends React.Component {