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 this._instance ? this._instance._instance : null;
};
var NoopInternalComponent = function(element) {
  this._renderedOutput = element;
  this._currentElement = element;
};
NoopInternalComponent.prototype = {
  mountComponent: function() {
  },
  receiveComponent: function(element) {
    this._renderedOutput = element;
    this._currentElement = element;
  },
  getNativeNode: function() {
    return undefined;
  },
  unmountComponent: function() {
  },
  getPublicInstance: function() {
    return null;
  },
};
var ShallowComponentWrapper = function(element) {
  this.construct(element);
};
Object.assign(
  ShallowComponentWrapper.prototype,
  ReactCompositeComponent.Mixin, {
    _instantiateReactComponent: function(element) {
      return new NoopInternalComponent(element);