Invalid downcast from interface

The following produces invalid cast:

```haxe
interface I {
function foo() : Int;
}

class Main implements I {

var x : Int = 66;

public function new() {
}

public function foo() {
return x;
}

static function test( x : Dynamic ) {
var p = Std.downcast(x,Main);
return p.x;
}

static function main() {
var x : I = new Main();
trace( test(x) );
}

}
```

This is because Std.downcast is implemented using `if( getType(x).check(Main) ) unsafe_cast(x) else null` and the unsafe cast does not get back the original value from the interface, meaning the returned value is the interface and not the class instance.