[vm/ffi] FfiNative optional arguments should be compile-time error

[dcharkes]

Optional arguments (positional and named) do not create compile-time errors:

@FfiNative<Double Function({Double foo})>('Math_sqrt', isLeaf:true)
external double _square_root(double x);

@FfiNative<Double Function([Double foo])>('Math_sqrt', isLeaf:true)
external double _square_root(double x);