**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)
getSnapshotBeforeUpdate() {
        throw Error('unexpected');
      }
      componentDidUpdate() {
        throw Error('unexpected');
      }
      render() {
        return null;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<Component value={1} />);
    shallowRenderer.render(<Component value={2} />);
  });
  it('should only render 1 level deep', () => {
    function Parent() {
      return (
        <div>
          <Child />
        </div>
      );
    }
    function Child() {
      throw Error('This component should not render');
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(React.createElement(Parent));
  });
  it('should have shallow rendering', () => {
    class SomeComponent extends React.Component {