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.

expect(console.error.calls.count()).toBe(2);
  });
  it('should not warn if mounting into non-empty node', function() {
    var container = document.createElement('container');
    container.innerHTML = '<div></div>';
    spyOn(console, 'error');
    ReactMount.render(<div />, container);
    expect(console.error.calls.count()).toBe(0);
  });
  it('should warn when mounting into document.body', function() {
    var iFrame = document.createElement('iframe');
    document.body.appendChild(iFrame);
    spyOn(console, 'error');
    ReactMount.render(<div />, iFrame.contentDocument.body);
    expect(console.error.calls.count()).toBe(1);
    expect(console.error.calls.argsFor(0)[0]).toContain(
      'Rendering components directly into document.body is discouraged'
    );
  });
  it('should account for escaping on a checksum mismatch', function() {
    var div = document.createElement('div');
    var markup = ReactDOMServer.renderToString(
      <div>This markup contains an nbsp entity: &nbsp; server text</div>);
    div.innerHTML = markup;
    spyOn(console, 'error');
    ReactDOM.render(
      <div>This markup contains an nbsp entity: &nbsp; client text</div>,
      div
    );
    expect(console.error.calls.count()).toBe(1);