get [Symbol.toStringTag]() : string and Object.prototype.toString.apply

So somewhere between computed property keys and changing the type of an object, the Symbol.toStringTag changes the output of an object that uses this defined key. By default Object.prototype.toString.apply can be used on any object to find an underlying type. Used on a regular expression it returns [object RegExp] or on a string it returns [object String].

On an object literal or even a class instance, it returns [object Object]. So the ES consortium came up with Symbol.toStringTag. When using this on an object, either via Object.defineProperty() or as a class getter with `get [Symbol.toStringTag]()`, you can coerce the type of the object to be something else.

```javascript
class Person {
get [Symbol.toStringTag]() {
return 'Person';
}
}

let person : Person = new Person();
let toString : Function = Object.prototype.toString;
console.log(toString.apply(person)); // '[object Person]'

let someObj : object = {};
console.log(toString.apply(someObj)); // '[object Object]'
Object.defineProperty(someObj, Symbol.toStringTag, {
get: function() : string {
return 'Person';
}
});
console.log(toString.apply(someObj)); // '[object Person]'
```

My questions are around how flowtype should and will evaluate objects or class instances that have a specified Symbol.toStringTag. IMHO it should report them as a class instance of the value returned by the getter.

Currently, none of this works with flowtype.