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).
current,
    workInProgress,
    renderExpirationTime,
  ) {
    pushHostContainer(workInProgress, workInProgress.stateNode.containerInfo);
    let nextChildren = workInProgress.pendingProps;
    if (hasContextChanged()) {
      
      
      if (nextChildren === null) {
        nextChildren = current && current.memoizedProps;
        invariant(
          nextChildren != 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 (
      nextChildren === null ||
      workInProgress.memoizedProps === nextChildren
    ) {
      return bailoutOnAlreadyFinishedWork(current, workInProgress);
    }
    if (current === null) {