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.

);
      var Link = React.createClass({
        render: function() {
          return <a>{this.props.children}</a>;
        },
      });
      ReactTestUtils.renderIntoDocument(<Link><div><Link /></div></Link>);
      expect(console.error.calls.count()).toBe(6);
      expect(console.error.calls.argsFor(5)[0]).toContain(
        'See Link > a > ... > Link > a.'
      );
    });
    it('should warn about incorrect casing on properties (ssr)', function() {
      spyOn(console, 'error');
      ReactDOMServer.renderToString(React.createElement('input', {type: 'text', tabindex: '1'}));
      expect(console.error.calls.count()).toBe(1);
      expect(console.error.calls.argsFor(0)[0]).toContain('tabIndex');
    });
    it('should warn about incorrect casing on event handlers (ssr)', function() {
      spyOn(console, 'error');
      ReactDOMServer.renderToString(React.createElement('input', {type: 'text', onclick: '1'}));
      ReactDOMServer.renderToString(React.createElement('input', {type: 'text', onKeydown: '1'}));
      expect(console.error.calls.count()).toBe(2);
      expect(console.error.calls.argsFor(0)[0]).toContain('onClick');
      expect(console.error.calls.argsFor(1)[0]).toContain('onKeyDown');
    });
    it('should warn about incorrect casing on properties', function() {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(React.createElement('input', {type: 'text', tabindex: '1'}));
      expect(console.error.calls.count()).toBe(1);