**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)
constructor(props, context) {
        super(props, context);
        this.state = {
          hasUpdatedState: false,
        };
      }
      UNSAFE_componentWillMount() {
        this.setState(
          {hasUpdatedState: true},
          () => (stateSuccessfullyUpdated = this.state.hasUpdatedState),
        );
      }
      render() {
        return <div>{this.props.children}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<Component />);
    expect(stateSuccessfullyUpdated).toBe(true);
  });
  it('should handle multiple callbacks', () => {
    const mockFn = jest.fn();
    const shallowRenderer = createRenderer();
    class Component extends React.Component {