Should we infer the type of expando properties

[phillipskevin]

Currently, this throws:

class Obj extends DefineObject {
  static get define() {
    return {
      age: 5
    };
  }
}

const obj = new Obj();
obj.age = "10"; // Uncaught Error: Type value '10' is not of type Number.

...but this doesn't:

class Obj extends DefineObject {
  static get define() {}
}

const obj = new Obj();
obj.age = 5;
obj.age = "10";

I think we should infer the type of expando properties the first time they're set.