optional args using polymorphic records + record field constraints

optional-args.js

// foo :: { 0: Num, 1: Maybe Num } -> Num
function foo(a, opt_b) {
    var b = opt_b == undefined ? 3 : opt_b;
    return a + b;
}

// foo here inferred to have this type:
// RowFields r Maybe => { 0: Num | r } -> b
foo(3); // translated to: foo { 0 = 3 | .. }