Old regression

I found that the following works since Haxe 4.2 while it didn't (as it should not) with oldest Haxe version:

```haxe
class Test {
static function main() {
var arr = [];
if( arr.length > 0 ) trace(arr[0].len++);
// we should have inferred arr as Array<{ len : Int, ... }> and not allow to push without this field defined
arr.push({ size : 0 });
}
}
```