[js] Using a StringMap in __init__

The `__init__` static of a StringMap is called after an `__init__` which uses a StringMap. Resulting in `__map_reserved` being undefined:

```
Cannot read property 'key' of undefined
```

Example (see compiled js): http://try.haxe.org/#241E8

``` haxe
class Test {
static function __init__() {
var a = new Map();
a.set('key', 'value');
trace(a.get('key'));
}
}
```