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() {
        throw new Error();
      }
    }
    var instance = <BadComponent />;
    expect(ReactCurrentOwner.current).toBe(null);
    expect(function() {
      instance = ReactTestUtils.renderIntoDocument(instance);
    }).toThrow();
    expect(ReactCurrentOwner.current).toBe(null);
  });
  it('should call componentWillUnmount before unmounting', () => {
    var container = document.createElement('div');
    var innerUnmounted = false;
    class Component extends React.Component {