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).
if (nextProps === null) {
        nextProps = memoizedProps;
      }
    } else {
      if (nextProps === null || memoizedProps === nextProps) {
        return bailoutOnAlreadyFinishedWork(current, workInProgress);
      }
      
      
    }
    var unmaskedContext = getUnmaskedContext(workInProgress);
    var context = getMaskedContext(workInProgress, unmaskedContext);
    var nextChildren;
    if (__DEV__) {
      ReactCurrentOwner.current = workInProgress;
      ReactDebugCurrentFiber.setCurrentPhase('render');
      nextChildren = fn(nextProps, context);
      ReactDebugCurrentFiber.setCurrentPhase(null);
    } else {
      nextChildren = fn(nextProps, context);
    }
    
    workInProgress.effectTag |= PerformedWork;
    reconcileChildren(current, workInProgress, nextChildren);
    memoizeProps(workInProgress, nextProps);
    return workInProgress.child;
  }
  function updateClassComponent(