**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)
static contextTypes = {
        name: PropTypes.string,
      };
      render() {
        return <div />;
      }
    }
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SimpleComponent />);
    expect(result).toEqual(<div />);
  });
  it('passes expected params to legacy component lifecycle methods', () => {
    const componentDidUpdateParams = [];
    const componentWillReceivePropsParams = [];
    const componentWillUpdateParams = [];
    const setStateParams = [];
    const shouldComponentUpdateParams = [];
    const initialProp = {prop: 'init prop'};
    const initialState = {state: 'init state'};
    const initialContext = {context: 'init context'};
    const updatedState = {state: 'updated state'};
    const updatedProp = {prop: 'updated prop'};
    const updatedContext = {context: 'updated context'};
    class SimpleComponent extends React.Component {