**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)
}
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SomeComponent />);
    expect(result.type).toBe('div');
    expect(result.props.children).toEqual([
      <span className="child1" />,
      <span className="child2" />,
    ]);
  });
  it('should handle ForwardRef', () => {
    const testRef = React.createRef();
    const SomeComponent = React.forwardRef((props, ref) => {
      expect(ref).toEqual(testRef);
      return (
        <div>
          <span className="child1" />
          <span className="child2" />
        </div>
      );
    });
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SomeComponent ref={testRef} />);
    expect(result.type).toBe('div');
    expect(result.props.children).toEqual([
      <span className="child1" />,
      <span className="child2" />,
    ]);
  });
  it('should handle Profiler', () => {
    class SomeComponent extends React.Component {