*feature*

>Warning: A component is `contentEditable` and contains `children` managed by React. It is now your responsibility to guarantee that none of those nodes are unexpectedly modified or duplicated. This is probably not intentional.

It would help heaps if I would know _which_ component is causing this.
spyOn(console, 'error');
      var defaultCreateElement = document.createElement.bind(document);
      try {
        document.createElement = element => {
          var container = defaultCreateElement(element);
          container.shadyRoot = {};
          return container;
        };
        mountComponent({is: 'custom-shady-div'});
        expectDev(console.error.calls.count()).toBe(1);
        expectDev(console.error.calls.argsFor(0)[0]).toContain(
          'A component is using shady DOM. Using shady DOM with React can ' +
            'cause things to break subtly.',
        );
        mountComponent({is: 'custom-shady-div2'});
        expectDev(console.error.calls.count()).toBe(1);
      } finally {
        document.createElement = defaultCreateElement;
      }
    });
    it('should treat menuitem as a void element but still create the closing tag', () => {
      
      spyOn(console, 'error');
      var container = document.createElement('div');
      var returnedValue = ReactDOMServer.renderToString(
        <menu><menuitem /></menu>,
      );
      expect(returnedValue).toContain('</menuitem>');
      expect(function() {
        ReactDOM.render(<menu><menuitem>children</menuitem></menu>, container);