**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 (
          <React.unstable_Profiler id="test" onRender={jest.fn()}>
            <div>
              <span className="child1" />
              <span className="child2" />
            </div>
          </React.unstable_Profiler>
        );
      }
    }
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SomeComponent />);
    expect(result.type).toBe(React.unstable_Profiler);
    expect(result.props.children).toEqual(
      <div>
        <span className="child1" />
        <span className="child2" />
      </div>,
    );
  });
  it('should enable shouldComponentUpdate to prevent a re-render', () => {
    let renderCounter = 0;
    class SimpleComponent extends React.Component {