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).
pushHostContext(workInProgress);
    if (current === null) {
      tryToClaimNextHydratableInstance(workInProgress);
    }
    const type = workInProgress.type;
    const memoizedProps = workInProgress.memoizedProps;
    let nextProps = workInProgress.pendingProps;
    if (nextProps === null) {
      nextProps = memoizedProps;
      invariant(
        nextProps !== null,
        'We should always have pending or current props. This error is ' +
          'likely caused by a bug in React. Please file an issue.',
      );
    }
    const prevProps = current !== null ? current.memoizedProps : null;
    if (hasContextChanged()) {
      
      
    } else if (nextProps === null || memoizedProps === nextProps) {
      return bailoutOnAlreadyFinishedWork(current, workInProgress);
    }