We should be able to look at see if the child has a data-react-id attribute but isn't rendered by this copy of React and consequently warn.

See https://groups.google.com/forum/#!topic/reactjs/mdwfKKJ0L2Q.

ReactDOM.unmountComponentAtNode(container);
    expect(Object.keys(ReactMount._instancesByReactRootID).length).toBe(1);
  });
  it('marks top-level mounts', function() {
    var ReactFeatureFlags = require('ReactFeatureFlags');
    class Foo extends React.Component {
      render() {
        return <Bar />;
      }
    }
    class Bar extends React.Component {
      render() {
        return <div />;
      }
    }
    try {
      ReactFeatureFlags.logTopLevelRenders = true;
      spyOn(console, 'time');
      spyOn(console, 'timeEnd');
      ReactTestUtils.renderIntoDocument(<Foo />);
      expect(console.time.calls.count()).toBe(1);
      expect(console.time.calls.argsFor(0)[0]).toBe('React mount: Foo');
      expect(console.timeEnd.calls.count()).toBe(1);
      expect(console.timeEnd.calls.argsFor(0)[0]).toBe('React mount: Foo');
    } finally {
      ReactFeatureFlags.logTopLevelRenders = false;
    }
  });
});