Set polyfill says it is [object Object] but should say [object Set]

Environment

Any - you can reproduce the issue using only file

https://github.com/facebook/react-native/blob/master/Libraries/vendor/core/Set.js

and any Javascript encironment.

EDIT: Just for the bot, it is completely irrelevant:

Environment:
OS: Windows 10
Node: 10.3.0
Yarn: Not Found
npm: 6.1.0
Watchman: Not Found
Xcode: N/A
Android Studio: Version 3.1.0.0 AI-173.4720617

Packages: (wanted => installed)
react: 16.3.1 => 16.3.1
react-native: 0.55.4 => 0.55.4

Description and Steps to Reproduce

Here is code that creates a simple Set object and then uses the spread operator to spread the Set's contents into a new array.

First, using an environment where the above file was executed/loaded (plus its own dependencies), and I also short-circuited the detection at the beginning that determines whether the polyfill is applied:

const s1 = new Set([1,2,3]);
console.log(Object.prototype.toString.call(s1));

Result:

[object Object]

Expected Behavior

Now for comparison a clean ES6+ environment without anything loaded, so we get a native Set implementation:

const s1 = new Set([1,2,3]);
console.log(Object.prototype.toString.call(s1));

Result:

[object Set]

Libraries/vendor/core/Set.js から始め、提供されている Object.prototype.toString.call の例を使って、報告された挙動を JavaScript 環境で再現します。polyfill の Set の結果とネイティブの Set の結果を比較します; Set インスタンスに対して polyfill が [object Set] を報告すれば完了です。