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).
expect(instance2.state.value).toBe(1);
  });
  it('should warn about `setState` in getChildContext', () => {
    spyOn(console, 'error');
    var container = document.createElement('div');
    var renderPasses = 0;
    class Component extends React.Component {
      state = {value: 0};
      getChildContext() {
        if (this.state.value === 0) {
          this.setState({value: 1});
        }
      }
      render() {
        renderPasses++;
        return <div />;
      }
    }
    Component.childContextTypes = {};
    expectDev(console.error.calls.count()).toBe(0);
    var instance = ReactDOM.render(<Component />, container);
    expect(renderPasses).toBe(2);
    expect(instance.state.value).toBe(1);
    expectDev(console.error.calls.count()).toBe(1);
    expectDev(console.error.calls.argsFor(0)[0]).toBe(
      'Warning: setState(...): Cannot call setState() inside getChildContext()',
    );
  });
  it('should cleanup even if render() fatals', () => {
    class BadComponent extends React.Component {