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(stubStyle.display).toEqual('block');
      expect(stubStyle.fontFamily).toEqual('Helvetica');
      expect(stubStyle.lineHeight).toEqual('1.2');
      styles.lineHeight = 0.5;
      ReactDOM.render(<div style={styles} />, container);
      expect(stubStyle.display).toEqual('block');
      expect(stubStyle.fontFamily).toEqual('Helvetica');
      expect(stubStyle.lineHeight).toEqual('0.5');
      ReactDOM.render(<div style={undefined} />, container);
      expect(stubStyle.display).toBe('');
      expect(stubStyle.fontFamily).toBe('');
      expect(stubStyle.lineHeight).toBe('');
    });
    it('should warn when mutating style', function() {
      spyOn(console, 'error');
      var style = {border: '1px solid black'};
      var App = React.createClass({
        getInitialState: function() {
          return {style: style};
        },
        render: function() {
          return <div style={this.state.style}>asd</div>;
        },
      });
      var stub = ReactTestUtils.renderIntoDocument(<App />);
      style.position = 'absolute';
      stub.setState({style: style});
      expect(console.error.calls.count()).toBe(1);
      expect(console.error.calls.argsFor(0)[0]).toEqual(
        'Warning: `div` was passed a style object that has previously been ' +