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,
      shouldUpdate,
      hasContext,
    );
  }
  function finishClassComponent(
    current: Fiber | null,
    workInProgress: Fiber,
    shouldUpdate: boolean,
    hasContext: boolean,
  ) {
    
    markRef(current, workInProgress);
    if (!shouldUpdate) {
      
      if (hasContext) {
        invalidateContextProvider(workInProgress, false);
      }
      return bailoutOnAlreadyFinishedWork(current, workInProgress);
    }
    const instance = workInProgress.stateNode;
    
    ReactCurrentOwner.current = workInProgress;
    let nextChildren;
    if (__DEV__) {
      ReactDebugCurrentFiber.setCurrentPhase('render');
      nextChildren = instance.render();
      ReactDebugCurrentFiber.setCurrentPhase(null);
    } else {