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).
workInProgress.expirationTime === NoWork ||
      workInProgress.expirationTime > renderExpirationTime
    ) {
      return bailoutOnLowPriority(current, workInProgress);
    }
    // 
    // 
    workInProgress.firstEffect = null;
    workInProgress.lastEffect = null;
    // 
    const nextChildren = null;
    reconcileChildrenAtExpirationTime(
      current,
      workInProgress,
      nextChildren,
      renderExpirationTime,
    );
    if (workInProgress.tag === ClassComponent) {
      const instance = workInProgress.stateNode;
      workInProgress.memoizedProps = instance.props;
      workInProgress.memoizedState = instance.state;
    }
    return workInProgress.child;
  }
  return {
    beginWork,
    beginFailedWork,
  };
};