To reproduce, open http://jsfiddle.net/0jjnaghw/
Error is logged to browser dev tools (in Chrome & Firefox, somehow not in Safari).

Happens on v15.0.0-rc.2 and on latest master version.

The assignment of additional event data to the newly created SyntheticEvent triggers a console error:
https://github.com/facebook/react/blob/3a4e1dbb5b2957875cab6af11a5e5934e125a0d8/src/test/ReactTestUtils.js#L502

> "Warning: This synthetic event is reused for performance reasons."

return <div />;
      },
    });
    var shallowRenderer = ReactTestUtils.createRenderer();
    expect(() => shallowRenderer.render(SomeComponent)).toThrow(
      'ReactShallowRenderer render(): Invalid component element. Instead of ' +
      'passing a component class, make sure to instantiate it by passing it ' +
      'to React.createElement.'
    );
    expect(() => shallowRenderer.render(<div />)).toThrow(
      'ReactShallowRenderer render(): Shallow rendering works only with ' +
      'custom components, not primitives (div). Instead of calling ' +
      '`.render(el)` and inspecting the rendered output, look at `el.props` ' +
      'directly instead.'
    );
  });
  it('should have shallow unmounting', function() {
    var componentWillUnmount = jest.genMockFn();
    var SomeComponent = React.createClass({
      render: function() {
        return <div />;
      },
      componentWillUnmount,
    });
    var shallowRenderer = ReactTestUtils.createRenderer();
    shallowRenderer.render(<SomeComponent />);
    shallowRenderer.unmount();
    expect(componentWillUnmount).toBeCalled();
  });
  it('can shallow render to null', function() {