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).
/**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */
'use strict';
import type {HostConfig} from 'react-reconciler';
import type {ReactCall} from 'shared/ReactTypes';
import type {Fiber} from 'react-reconciler/src/ReactFiber';
import type {HostContext} from './ReactFiberHostContext';
import type {HydrationContext} from './ReactFiberHydrationContext';
import type {FiberRoot} from './ReactFiberRoot';
import type {ExpirationTime} from './ReactFiberExpirationTime';
var {
  IndeterminateComponent,
  FunctionalComponent,
  ClassComponent,
  HostRoot,
  HostComponent,