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:

constructor(props, context) {
        super(props, context);
        this.setState({stateField: 'something'});
        this.state = {stateField: 'somethingelse'};
      }
      render() {
        return <div />;
      }
    }
    expect(() => {
      ReactTestUtils.renderIntoDocument(<StatefulComponent />);
    }).toWarnDev(
      "Warning: Can't call setState on a component that is not yet mounted. " +
        'This is a no-op, but it might indicate a bug in your application. ' +
        'Instead, assign to `this.state` directly or define a `state = {};` ' +
        'class property with the desired state in the StatefulComponent component.',
      {withoutStack: true},
    );
    
    ReactTestUtils.renderIntoDocument(<StatefulComponent />);
  });
  it('should correctly determine if a component is mounted', () => {
    class Component extends React.Component {