When you forget to extend `React.Component` you get the following error:

```
TypeError: Cannot call a class as a function
    at _classCallCheck (null.js:7:99)
    at Hello (null.js:11:5)
    ...react internals...
```

[[Example](http://react.jsbin.com/?html,js,console)]

It would be nice if in dev you could do a check for `Component.prototype.render` and if it exists, provide a better warning along the lines of:

```
Warning: Using a class with a render method as a function, did you forget to extend React.Component?
``` 

I believe it would happen in [ReactCompositeComponent.js](https://github.com/facebook/react/blob/b840229286ac2a82fa49553ce793cf7b953d1845/src/renderers/shared/stack/reconciler/ReactCompositeComponent.js#L248).
render() {
        return <Spy key={this.props.name} name={this.props.name} />;
      }
    }
    var container = document.createElement('div');
    ReactDOM.render(<Wrapper name="A" />, container);
    ReactDOM.render(<Wrapper name="B" />, container);
    expect(log).toEqual([
      'A componentWillMount',
      'A render',
      'A componentDidMount',
      'B componentWillMount',
      'B render',
      'A componentWillUnmount',
      'B componentDidMount',
    ]);
  });
  it('respects a shallow shouldComponentUpdate implementation', () => {
    var renderCalls = 0;
    class PlasticWrap extends React.Component {