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).
return bailoutOnLowPriority(current, workInProgress);
    }
    switch (workInProgress.tag) {
      case IndeterminateComponent:
        return mountIndeterminateComponent(
          current,
          workInProgress,
          renderExpirationTime,
        );
      case FunctionalComponent:
        return updateFunctionalComponent(current, workInProgress);
      case ClassComponent:
        return updateClassComponent(
          current,
          workInProgress,
          renderExpirationTime,
        );
      case HostRoot:
        return updateHostRoot(current, workInProgress, renderExpirationTime);
      case HostComponent:
        return updateHostComponent(
          current,
          workInProgress,
          renderExpirationTime,
        );
      case HostText:
        return updateHostText(current, workInProgress);
      case CallHandlerPhase:
        
        workInProgress.tag = CallComponent;