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).
constructor(props) {
        var _props = {idx: props.idx + '!'};
        super(_props);
      }
      render() {
        return <span />;
      }
    }
    expectDev(console.error.calls.count()).toBe(0);
    ReactDOM.render(<Foo idx="qwe" />, container);
    expectDev(console.error.calls.count()).toBe(1);
    expectDev(console.error.calls.argsFor(0)[0]).toContain(
      'Foo(...): When calling super() in `Foo`, make sure to pass ' +
        "up the same props that your component's constructor was passed.",
    );
  });
  it('should only call componentWillUnmount once', () => {
    var app;
    var count = 0;
    class App extends React.Component {
      render() {
        if (this.props.stage === 1) {
          return <UnunmountableComponent />;
        } else {
          return null;
        }
      }
    }
    class UnunmountableComponent extends React.Component {