Here at lines 26 and 27 are type errors.

sasai-kudasai.js

// @flow

function MSasaiable (constructor) {
    return class extends constructor {
        sasai (string : string) {
            return `Sasai ${string}!`;
        }
    };
}

function MKudasaiable (constructor) {
    return class extends constructor {
        kudasai (string : string) {
            return `Kudasai ${string}.`;
        }
    };
}

function MSasaiableKudasaiable (constructor) {
    return MSasaiable(MKudasaiable(constructor));
}

const SasaiableKudasaiable = MSasaiableKudasaiable(class {});

const sasaiableKudasaiable = new SasaiableKudasaiable();
console.log(sasaiableKudasaiable.sasai(1));
console.log(sasaiableKudasaiable.kudasai(null));