将mixins添加到Flow中的类

Question

我可以将mixins添加到函数原型中，但似乎我无法使用类原型来实现它：

例如code

var mixins = { doSmth() {} }

function F() {}
Object.assign(F.prototype, mixins);
new F().doSmth();

class C {}
Object.assign(C.prototype, mixins);
new C().doSmth();

会导致这些错误

8: Object.assign(C.prototype, mixins);
   ^ property `doSmth` of object literal. Property not found in
8: Object.assign(C.prototype, mixins);
                 ^ C

9: new C().doSmth();
           ^ property `doSmth`. Property not found in
9: new C().doSmth();
   ^ C

如何将mixins添加到类中？