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 transaction = ReactUpdates.ReactReconcileTransaction.getPooled(true);
  renderer._render(element, transaction, context);
  ReactUpdates.ReactReconcileTransaction.release(transaction);
}
ReactShallowRenderer.prototype.getRenderOutput = function() {
  return (
    (this._instance && this._instance._renderedComponent &&
     this._instance._renderedComponent._renderedOutput)
    || null
  );
};
ReactShallowRenderer.prototype.unmount = function() {
  if (this._instance) {
    this._instance.unmountComponent(false);
  }
};
ReactShallowRenderer.prototype._render = function(element, transaction, context) {
  if (this._instance) {
    this._instance.receiveComponent(element, transaction, context);
  } else {