Feature - use correct Event subclasses for all kinds of events

Given the code -

``` js
window.addEventListener("message", handler, false);
```

`handler` would be called with a `MessageEvent` argument.

Closure Compiler assumes that any `handler` in `addEventListener` takes an `Event` argument, which is incorrect and leads to false warnings (see #506).

Perhaps Closure Compiler can infer the type of event using the event type argument?

Of course, this only works for explicit cases like the one mentioned above, but will not work for other cases like a generic event listener adding function (like `$.fn.on`) - but I think this is fine.

(Same for `attachEvent`, I guess)

After all, more type checking leads to better warnings which leads to less bugs.

This could be implemented as JSDocs maybe, or as EventExterns of some sort that parses this -

``` js
/** @param {string} type
@param {function(!MessageEvent)} handler
@param {boolean=} opt_capture */
Window.prototype.addEventListener = function (type="message", handler, opt_capture);
```

Just an idea, there are probably other ways or syntax.