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:

_isMounted() {
        
        
        return this.updater.isMounted(this);
      }
      UNSAFE_componentWillMount() {
        expect(this._isMounted()).toBeFalsy();
      }
      componentDidMount() {
        expect(this._isMounted()).toBeTruthy();
      }
      render() {
        expect(this._isMounted()).toBeFalsy();
        return <div />;
      }
    }
    const element = <Component />;
    expect(() => {
      const instance = ReactTestUtils.renderIntoDocument(element);
      expect(instance._isMounted()).toBeTruthy();
    }).toWarnDev('Component is accessing isMounted inside its render()', {
      withoutStack: true,
    });
  });
  it('should correctly determine if a null component is mounted', () => {
    class Component extends React.Component {