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).
pushHostRootContext(workInProgress);
    const updateQueue = workInProgress.updateQueue;
    if (updateQueue !== null) {
      const prevState = workInProgress.memoizedState;
      const state = processUpdateQueue(
        current,
        workInProgress,
        updateQueue,
        null,
        null,
        renderExpirationTime,
      );
      if (prevState === state) {
        
        
        resetHydrationState();
        return bailoutOnAlreadyFinishedWork(current, workInProgress);
      }
      const element = state.element;
      const root: FiberRoot = workInProgress.stateNode;
      if (
        (current === null || current.child === null) &&
        root.hydrate &&
        enterHydrationState(workInProgress)
      ) {