**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)
}
    SomeComponent.contextTypes = {
      bar: PropTypes.string,
    };
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SomeComponent foo={'FOO'} />, {
      bar: 'BAR',
    });
    expect(result.type).toBe('div');
    expect(result.props.children).toEqual([
      <div>FOO</div>,
      <div>BAR</div>,
      <span className="child1" />,
      <span className="child2" />,
    ]);
  });
  it('should shallow render a component returning strings directly from render', () => {
    const Text = ({value}) => value;
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<Text value="foo" />);
    expect(result).toEqual('foo');
  });
  it('should shallow render a component returning numbers directly from render', () => {
    const Text = ({value}) => value;
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<Text value={10} />);
    expect(result).toEqual(10);
  });
  it('should shallow render a fragment', () => {
    class SomeComponent extends React.Component {