Currently, if you define `shouldComponentUpdate` on `React.PureComponent` nothing bad happens and it acts like `React.Component` https://jsfiddle.net/jekmdfva/

We should probably warn in dev since it defaults the point of `PureComponent`
render() {
        renders++;
        return <div />;
      }
      shouldComponentUpdate() {
        return true;
      }
    }
    var container = document.createElement('div');
    ReactDOM.render(<Component />, container);
    ReactDOM.render(<Component />, container);
    expect(renders).toBe(2);
  });
  it('extends React.Component', () => {
    var renders = 0;
    class Component extends React.PureComponent {
      render() {
        expect(this instanceof React.Component).toBe(true);
        expect(this instanceof React.PureComponent).toBe(true);
        renders++;
        return <div />;
      }
    }
    ReactDOM.render(<Component />, document.createElement('div'));
    expect(renders).toBe(1);
  });
});