It is a Jest-ism that shouldn't be in our open source package.
People using Jest can implement a similar pattern directly.
render() {
        return (
          <div>
            Hello <span className={'x y z'}>Jim</span>
          </div>
        );
      }
    }
    const renderedComponent = ReactTestUtils.renderIntoDocument(<Wrapper />);
    const scryResults1 = ReactTestUtils.scryRenderedDOMComponentsWithClass(
      renderedComponent,
      'x y',
    );
    expect(scryResults1.length).toBe(1);
    const scryResults2 = ReactTestUtils.scryRenderedDOMComponentsWithClass(
      renderedComponent,
      'x z',
    );
    expect(scryResults2.length).toBe(1);
    const scryResults3 = ReactTestUtils.scryRenderedDOMComponentsWithClass(
      renderedComponent,
      ['x', 'y'],
    );
    expect(scryResults3.length).toBe(1);
    expect(scryResults1[0]).toBe(scryResults2[0]);
    expect(scryResults1[0]).toBe(scryResults3[0]);
    const scryResults4 = ReactTestUtils.scryRenderedDOMComponentsWithClass(
      renderedComponent,