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.

'mutated. Mutating `style` is deprecated. Consider cloning it ' +
        'beforehand. Check the `render` of `App`. Previous style: ' +
        '{border: "1px solid black"}. Mutated style: ' +
        '{border: "1px solid black", position: "absolute"}.'
      );
      style = {background: 'red'};
      stub = ReactTestUtils.renderIntoDocument(<App />);
      style.background = 'green';
      stub.setState({style: {background: 'green'}});
      
      expect(console.error.calls.count()).toBe(1);
      style = {background: 'red'};
      var div = document.createElement('div');
      ReactDOM.render(<span style={style}></span>, div);
      style.background = 'blue';
      ReactDOM.render(<span style={style}></span>, div);
      expect(console.error.calls.count()).toBe(2);
    });
    it('should warn for unknown prop', function() {
      spyOn(console, 'error');
      var container = document.createElement('div');
      ReactDOM.render(<div foo="bar" />, container);
      expect(console.error.calls.count(0)).toBe(1);
      expect(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Unknown prop `foo` on <div> tag. Remove this prop from the element. ' +
        'For details, see https://fb.me/react-unknown-prop\n    in div (at **)'
      );
    });
    it('should warn for onDblClick prop', function() {
      spyOn(console, 'error');