**Do you want to request a *feature* or report a *bug*?**
bug

**What is the current behavior?**
In normal rendering, you can memoize a class-based component: https://jsfiddle.net/586ea3cx/

With the shallow renderer, it seems like you can't: https://jsfiddle.net/odj217Lv/1/

This might be blocking https://github.com/airbnb/enzyme/pull/1914, in which tests are failing with `Cannot call a class as a function`. (it's tough to repro stuff with the shallow renderer)
render() {
        return <div />;
      }
    }
    class Fragment extends React.Component {
      render() {
        return (
          <React.Fragment>
            <div />
            <span />
            <SomeComponent />
          </React.Fragment>
        );
      }
    }
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<Fragment />);
    expect(result).toEqual(
      <React.Fragment>
        <div />
        <span />
        <SomeComponent />
      </React.Fragment>,
    );
  });
  it('should throw for invalid elements', () => {
    class SomeComponent extends React.Component {