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).
var nextChildren = workInProgress.pendingProps;
    if (hasContextChanged()) {
      
      
      if (nextChildren === null) {
        nextChildren = workInProgress.memoizedProps;
      }
    } else if (
      nextChildren === null ||
      workInProgress.memoizedProps === nextChildren
    ) {
      return bailoutOnAlreadyFinishedWork(current, workInProgress);
    }
    reconcileChildren(current, workInProgress, nextChildren);
    memoizeProps(workInProgress, nextChildren);
    return workInProgress.child;
  }
  function markRef(current: Fiber | null, workInProgress: Fiber) {
    const ref = workInProgress.ref;
    if (ref !== null && (!current || current.ref !== ref)) {
      
      workInProgress.effectTag |= Ref;
    }
  }
  function updateFunctionalComponent(current, workInProgress) {
    var fn = workInProgress.type;
    var nextProps = workInProgress.pendingProps;
    const memoizedProps = workInProgress.memoizedProps;
    if (hasContextChanged()) {