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).
state = {value: 0};
      render() {
        renderPasses++;
        renderedState = this.state.value;
        if (this.state.value === 0) {
          this.setState({value: 1});
        }
        return <div />;
      }
    }
    expectDev(console.error.calls.count()).toBe(0);
    var instance = ReactDOM.render(<Component />, container);
    expectDev(console.error.calls.count()).toBe(1);
    expectDev(console.error.calls.argsFor(0)[0]).toContain(
      'Cannot update during an existing state transition (such as within ' +
        "`render` or another component's constructor). Render methods should " +
        'be a pure function of props and state; constructor side-effects are ' +
        'an anti-pattern, but can be moved to `componentWillMount`.',
    );
    
    
    
    expect(renderPasses).toBe(2);
    expect(renderedState).toBe(1);
    expect(instance.state.value).toBe(1);
    
    var instance2 = ReactDOM.render(<Component prop={123} />, container);
    expect(instance).toBe(instance2);
    expect(renderedState).toBe(1);