**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>{this.context.name}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SimpleComponent />, {
      name: 'foo',
    });
    expect(result).toEqual(<div>foo</div>);
  });
  it('should track context across updates', () => {
    class SimpleComponent extends React.Component {