Type inference is not working with callback function parameters.

JSX can detect type automatically from function parameter, but if parameter is callback function, it can't search grand child parameters.

``` js
// definition
class async {
    static function only_once.<T>(fn : (T) -> void) : (T) -> void { 
        return ...
    }
}

// user code
// JSX can't detect T == int
async.only_once((a : int) -> {
});
```
