This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
expect(function() {
      ReactTestUtils.renderIntoDocument(
        <div>
          <NotAComponent />
        </div>,
      );
    }).toThrowError(
      'NotAComponent(...): Nothing was returned from render. ' +
        'This usually means a return statement is missing. Or, to render nothing, return null.',
    );
  });
  it('should throw on string refs in pure functions', () => {
    function Child() {