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).
renderExpirationTime: ExpirationTime,
  ) {
    
    switch (workInProgress.tag) {
      case ClassComponent:
        pushContextProvider(workInProgress);
        break;
      case HostRoot:
        pushHostRootContext(workInProgress);
        break;
      default:
        invariant(
          false,
          'Invalid type of work. This error is likely caused by a bug in React. ' +
            'Please file an issue.',
        );
    }
    
    workInProgress.effectTag |= Err;
    
    // 
    // 
    // 
    // 
    if (current === null) {
      workInProgress.child = null;
    } else if (workInProgress.child !== current.child) {
      workInProgress.child = current.child;
    }
    if (