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: Fiber | null,
    workInProgress: Fiber,
    renderExpirationTime: ExpirationTime,
  ) {
    
    
    
    const hasContext = pushContextProvider(workInProgress);
    let shouldUpdate;
    if (current === null) {
      if (!workInProgress.stateNode) {
        
        constructClassInstance(workInProgress, workInProgress.pendingProps);
        mountClassInstance(workInProgress, renderExpirationTime);
        shouldUpdate = true;
      } else {
        invariant(false, 'Resuming work not yet implemented.');
        
        
      }
    } else {
      shouldUpdate = updateClassInstance(
        current,
        workInProgress,
        renderExpirationTime,
      );
    }
    return finishClassComponent(