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.

render() {
        return <div><div /></div>;
      }
    }
    ReactDOM.render(<Component />, container);
    
    spyOn(console, 'error');
    var rootNode = container.firstChild;
    ReactDOM.render(<span />, rootNode);
    expect(console.error.calls.count()).toBe(1);
    expect(console.error.calls.argsFor(0)[0]).toBe(
      'Warning: render(...): Replacing React-rendered children with a new ' +
      'root component. If you intended to update the children of this node, ' +
      'you should instead have the existing children update their state and ' +
      'render the new components instead of calling ReactDOM.render.'
    );
  });
  it('passes the correct callback context', function() {
    var container = document.createElement('div');
    var calls = 0;
    ReactDOM.render(<div />, container, function() {
      expect(this.nodeName).toBe('DIV');
      calls++;
    });
    
    ReactDOM.render(<div />, container, function() {
      expect(this.nodeName).toBe('DIV');
      calls++;