The error event on `<source>` elements doesn't work. Looking at #4370, it seems like there is an `onError` for `<video>` but I can't tell if that's supposed to work for `<source>` too.

The error event is useful so I can deduce which format the browser is using.

expect(
        normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])
      ).toBe(
        'Warning: Unknown DOM property class. Did you mean className?\n    in div (at **)'
      );
      expect(
        normalizeCodeLocInfo(console.error.calls.argsFor(1)[0])
      ).toBe(
        'Warning: Unknown event handler property onclick. Did you mean ' +
        '`onClick`?\n    in input (at **)'
      );
    });
    it('gives source code refs for unknown prop warning for update render', function() {
      spyOn(console, 'error');
      var container = document.createElement('div');
      ReactDOMServer.renderToString(<div className="paladin" />, container);
      expect(console.error.calls.count()).toBe(0);
      ReactDOMServer.renderToString(<div class="paladin" />, container);
      expect(console.error.calls.count()).toBe(1);
      expect(
        normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])
      ).toBe(
        'Warning: Unknown DOM property class. Did you mean className?\n    in div (at **)'
      );
    });
    it('gives source code refs for unknown prop warning for exact elements ', function() {
      spyOn(console, 'error');
      ReactDOMServer.renderToString(
        <div className="foo1">
        <div class="foo2"/>