[New] shim/auto: add flat to Symbol.unscopables

Author: ljharb

package.json

@@ -49,7 +49,8 @@
 	"dependencies": {
 		"call-bind": "^1.0.2",
 		"define-properties": "^1.1.3",
-		"es-abstract": "^1.19.2"
+		"es-abstract": "^1.19.2",
+		"es-shim-unscopables": "^1.0.0"
 	},
 	"devDependencies": {
 		"@es-shims/api": "^2.2.3",

shim.js

@@ -1,14 +1,20 @@
 'use strict';
 
 var define = require('define-properties');
+var shimUnscopables = require('es-shim-unscopables');
+
 var getPolyfill = require('./polyfill');
 
 module.exports = function shimFlat() {
 	var polyfill = getPolyfill();
+
 	define(
 		Array.prototype,
 		{ flat: polyfill },
 		{ flat: function () { return Array.prototype.flat !== polyfill; } }
 	);
+
+	shimUnscopables('flat');
+
 	return polyfill;
 };