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 = {x: 1};
      render() {
        return <div />;
      }
    }
    var moo = ReactTestUtils.renderIntoDocument(<Moo />);
    
    
    moo.updater.enqueueReplaceState(moo, {y: 2});
    expect('x' in moo.state).toBe(false);
    expect(moo.state.y).toBe(2);
  });
  it('should support objects with prototypes as state', () => {
    var NotActuallyImmutable = function(str) {
      this.str = str;
    };
    NotActuallyImmutable.prototype.amIImmutable = function() {
      return true;
    };
    class Moo extends React.Component {
      state = new NotActuallyImmutable('first');
      
      
      _replaceState = update => this.updater.enqueueReplaceState(this, update);
      render() {
        return <div />;
      }
    }
    var moo = ReactTestUtils.renderIntoDocument(<Moo />);
    expect(moo.state.str).toBe('first');