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).
HostText,
  HostPortal,
  CallComponent,
  CallHandlerPhase,
  ReturnComponent,
  Fragment,
} = require('shared/ReactTypeOfWork');
var {
  PerformedWork,
  Placement,
  ContentReset,
  Err,
  Ref,
} = require('shared/ReactTypeOfSideEffect');
var {ReactCurrentOwner} = require('shared/ReactGlobalSharedState');
var invariant = require('fbjs/lib/invariant');
var ReactFiberClassComponent = require('./ReactFiberClassComponent');
var {
  mountChildFibersInPlace,
  reconcileChildFibers,
  reconcileChildFibersInPlace,
  cloneChildFibers,
} = require('./ReactChildFiber');
var {processUpdateQueue} = require('./ReactFiberUpdateQueue');
var {
  getMaskedContext,
  getUnmaskedContext,
  hasContextChanged,
  pushContextProvider,
  pushTopLevelContextObject,