*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.
React.createElement('svg', null, React.createElement('PATH')),
      );
      expectDev(console.error.calls.count()).toBe(0);
      ReactTestUtils.renderIntoDocument(React.createElement('CUSTOM-TAG'));
      expectDev(console.error.calls.count()).toBe(0);
      ReactTestUtils.renderIntoDocument(React.createElement('IMG'));
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        '<IMG /> is using uppercase HTML.',
      );
    });
    it('should warn on props reserved for future use', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(<div aria="hello" />);
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'The `aria` attribute is reserved for future use in React. ' +
          'Pass individual `aria-` attributes instead.',
      );
    });
    it('should warn if the tag is unrecognized', () => {
      spyOn(console, 'error');
      let realToString;
      try {
        realToString = Object.prototype.toString;
        let wrappedToString = function() {
          
          if (this instanceof window.HTMLUnknownElement) {
            return '[object HTMLUnknownElement]';
          }