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).
invalidateContextProvider,
} = require('./ReactFiberContext');
var {NoWork, Never} = require('./ReactFiberExpirationTime');
if (__DEV__) {
  var warning = require('fbjs/lib/warning');
  var ReactDebugCurrentFiber = require('./ReactDebugCurrentFiber');
  var {cancelWorkTimer} = require('./ReactDebugFiberPerf');
  var warnedAboutStatelessRefs = {};
}
module.exports = function<T, P, I, TI, PI, C, CC, CX, PL>(
  config: HostConfig<T, P, I, TI, PI, C, CC, CX, PL>,
  hostContext: HostContext<C, CX>,
  hydrationContext: HydrationContext<C, CX>,
  scheduleWork: (fiber: Fiber, expirationTime: ExpirationTime) => void,
  computeExpirationForFiber: (fiber: Fiber) => ExpirationTime,
) {
  const {
    shouldSetTextContent,
    useSyncScheduling,
    shouldDeprioritizeSubtree,
  } = config;
  const {pushHostContext, pushHostContainer} = hostContext;
  const {
    enterHydrationState,
    resetHydrationState,
    tryToClaimNextHydratableInstance,
  } = hydrationContext;
  const {
    adoptClassInstance,
    constructClassInstance,