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 container = document.createElement('div');
      ReactDOM.render(<div onDblClick={() => {}} />, container);
      expect(console.error.calls.count(0)).toBe(1);
      expect(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Unknown event handler property onDblClick. Did you mean `onDoubleClick`?\n    in div (at **)'
      );
    });
    it('should not warn for "0" as a unitless style value', function() {
      spyOn(console, 'error');
      var Component = React.createClass({
        render: function() {
          return <div style={{margin: '0'}} />;
        },
      });
      ReactTestUtils.renderIntoDocument(<Component />);
      expect(console.error.calls.count()).toBe(0);
    });
    it('should warn nicely about NaN in style', function() {
      spyOn(console, 'error');
      var style = {fontSize: NaN};
      var div = document.createElement('div');
      ReactDOM.render(<span style={style}></span>, div);
      ReactDOM.render(<span style={style}></span>, div);
      expect(console.error.calls.count()).toBe(1);
      expect(console.error.calls.argsFor(0)[0]).toEqual(
        'Warning: `NaN` is an invalid value for the `fontSize` css style property.',
      );
    });
    it('should update styles if initially null', function() {
      var styles = null;