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).
var nextCall = (workInProgress.pendingProps: null | ReactCall);
    if (hasContextChanged()) {
      
      
      if (nextCall === null) {
        nextCall = current && current.memoizedProps;
        invariant(
          nextCall !== null,
          'We should always have pending or current props. This error is ' +
            'likely caused by a bug in React. Please file an issue.',
        );
      }
    } else if (nextCall === null || workInProgress.memoizedProps === nextCall) {
      nextCall = workInProgress.memoizedProps;
      
      
      
    }
    const nextChildren = nextCall.children;