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).
componentWillUnmount() {
        app.setState({});
        count++;
        throw Error('always fails');
      }
      render() {
        return <div>Hello {this.props.name}</div>;
      }
    }
    var container = document.createElement('div');
    var setRef = ref => {
      if (ref) {
        app = ref;
      }
    };
    expect(function() {
      ReactDOM.render(<App ref={setRef} stage={1} />, container);
      ReactDOM.render(<App ref={setRef} stage={2} />, container);
    }).toThrow();
    expect(count).toBe(1);
  });
  it('prepares new child before unmounting old', () => {
    var log = [];
    class Spy extends React.Component {