It seems problematic to allow arbitary non-lowercase attribute on HTML nodes. Because the server has no ability to keep them case sensitive (HTML isn't). So they will end up different on the client.

We should probably warn if the attribute is not a known one *and* is not lowercase.
expectDev(console.error.calls.count()).toBe(0);
      ReactTestUtils.renderIntoDocument(<div onfocusin={() => {}} />);
      expectDev(console.error.calls.count()).toBe(1);
      ReactTestUtils.renderIntoDocument(<div onfocusout={() => {}} />);
      expectDev(console.error.calls.count()).toBe(2);
    });
    it('should warn about props that are no longer supported (ssr)', () => {
      spyOn(console, 'error');
      ReactDOMServer.renderToString(<div />);
      expectDev(console.error.calls.count()).toBe(0);
      ReactDOMServer.renderToString(<div onFocusIn={() => {}} />);
      expectDev(console.error.calls.count()).toBe(1);
      ReactDOMServer.renderToString(<div onFocusOut={() => {}} />);
      expectDev(console.error.calls.count()).toBe(2);
    });
    it('should warn about props that are no longer supported without case sensitivity (ssr)', () => {
      spyOn(console, 'error');
      ReactDOMServer.renderToString(<div />);
      expectDev(console.error.calls.count()).toBe(0);
      ReactDOMServer.renderToString(<div onfocusin={() => {}} />);
      expectDev(console.error.calls.count()).toBe(1);
      ReactDOMServer.renderToString(<div onfocusout={() => {}} />);
      expectDev(console.error.calls.count()).toBe(2);
    });
    it('gives source code refs for unknown prop warning', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(<div class="paladin" />);
      ReactTestUtils.renderIntoDocument(<input type="text" onclick="1" />);
      expectDev(console.error.calls.count()).toBe(2);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(