default parameter with other param

This code fails to compile.
```
class _Main {
    static function fn(a : number, b : number = a * 2) : number {
        return a + b;
    }

    static function main(args : string[]) : void {
        log _Main.fn(5);
    }
}
```

```
ERROR!
[input:2:48] could not find definition for template class: 'a'
    static function fn(a : number, b : number = a * 2) : number {
                                                ^
[input:2:48] no class definition or variable for 'a'
    static function fn(a : number, b : number = a * 2) : number {
                                                ^
```

In ECMAScript6 of Firefox, allow default parameters to be initialized with other parameter value.
How do you feel about that?