**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)
state = {clicked: false};
      handleUserClick = () => {
        this.setState({clicked: true});
      };
      render() {
        return (
          <div
            ref={() => {}}
            onClick={this.handleUserClick}
            className={this.state.clicked ? 'clicked' : ''}
          />
        );
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<SimpleComponent />);
    let result = shallowRenderer.getRenderOutput();
    expect(result.type).toEqual('div');
    expect(result.props.className).toEqual('');
    result.props.onClick();
    result = shallowRenderer.getRenderOutput();
    expect(result.type).toEqual('div');
    expect(result.props.className).toEqual('clicked');
  });
  it('can initialize state via static getDerivedStateFromProps', () => {
    class SimpleComponent extends React.Component {
      state = {
        count: 1,
      };
      static getDerivedStateFromProps(props, prevState) {