It is a Jest-ism that shouldn't be in our open source package.
People using Jest can implement a similar pattern directly.
handleClick = e => {
          expect(e.clientX).toBe(CLIENT_X);
        };
        render() {
          return <div onClick={this.handleClick} />;
        }
      }
      const element = document.createElement('div');
      const instance = ReactDOM.render(<Component />, element);
      ReactTestUtils.Simulate.click(ReactDOM.findDOMNode(instance), {
        clientX: CLIENT_X,
      });
    });
    it('should set the type of the event', () => {
      let event;
      const stub = jest.fn().mockImplementation(e => {
        e.persist();
        event = e;
      });
      const container = document.createElement('div');
      const instance = ReactDOM.render(<div onKeyDown={stub} />, container);
      const node = ReactDOM.findDOMNode(instance);
      ReactTestUtils.Simulate.keyDown(node);
      expect(event.type).toBe('keydown');
      expect(event.nativeEvent.type).toBe('keydown');
    });
    it('should work with renderIntoDocument', () => {
      const onChange = jest.fn();
      class MyComponent extends React.Component {