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."

var SomeComponent = React.createClass({
      render: function() {
        return null;
      },
    });
    var shallowRenderer = ReactTestUtils.createRenderer();
    var result = shallowRenderer.render(<SomeComponent />);
    expect(result).toBe(null);
  });
  it('can shallow render with a ref', function() {
    var SomeComponent = React.createClass({
      render: function() {
        return <div ref="hello" />;
      },
    });
    var shallowRenderer = ReactTestUtils.createRenderer();
    
    shallowRenderer.render(<SomeComponent />);
  });
  it('lets you update shallowly rendered components', function() {
    var SomeComponent = React.createClass({
      getInitialState: function() {
        return {clicked: false};
      },
      onClick: function() {
        this.setState({clicked: true});
      },
      render: function() {
        var className = this.state.clicked ? 'was-clicked' : '';
        if (this.props.aNew === 'prop') {