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).
nextChildren = instance.render();
    }
    
    workInProgress.effectTag |= PerformedWork;
    reconcileChildren(current, workInProgress, nextChildren);
    
    
    memoizeState(workInProgress, instance.state);
    memoizeProps(workInProgress, instance.props);
    
    if (hasContext) {
      invalidateContextProvider(workInProgress, true);
    }
    return workInProgress.child;
  }
  function pushHostRootContext(workInProgress) {
    const root = (workInProgress.stateNode: FiberRoot);
    if (root.pendingContext) {
      pushTopLevelContextObject(
        workInProgress,
        root.pendingContext,
        root.pendingContext !== root.context,
      );
    } else if (root.context) {
      
      pushTopLevelContextObject(workInProgress, root.context, false);
    }
    pushHostContainer(workInProgress, root.containerInfo);
  }
  function updateHostRoot(current, workInProgress, renderExpirationTime) {