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."

});
  it('should support injected wrapper components as DOM components', function() {
    var getTestDocument = require('getTestDocument');
    var injectedDOMComponents = [
      'button',
      'form',
      'iframe',
      'img',
      'input',
      'option',
      'select',
      'textarea',
    ];
    injectedDOMComponents.forEach(function(type) {
      var testComponent = ReactTestUtils.renderIntoDocument(
        React.createElement(type)
      );
      expect(testComponent.tagName).toBe(type.toUpperCase());
      expect(ReactTestUtils.isDOMComponent(testComponent)).toBe(true);
    });
    
    
    var Root = React.createClass({
      render: function() {
        return (
          <html ref="html">
            <head ref="head">
              <title>hello</title>
            </head>
            <body ref="body">