[Fix] avoid mutating Object.assign in modern engines

index.js

@@ -1,17 +1,22 @@
 'use strict';
 
 var defineProperties = require('define-properties');
+var callBind = require('es-abstract/helpers/callBind');
 
 var implementation = require('./implementation');
 var getPolyfill = require('./polyfill');
 var shim = require('./shim');
 
-var polyfill = getPolyfill();
+var polyfill = callBind.apply(getPolyfill());
+// eslint-disable-next-line no-unused-vars
+var bound = function assign(target, source1) {
+	return polyfill(Object, arguments);
+};
 
-defineProperties(polyfill, {
+defineProperties(bound, {
 	getPolyfill: getPolyfill,
 	implementation: implementation,
 	shim: shim
 });
 
-module.exports = polyfill;
+module.exports = bound;

package.json

@@ -9,7 +9,7 @@
 	"license": "MIT",
 	"main": "index.js",
 	"scripts": {
-		"pretest": "npm run --silent lint && es-shim-api",
+		"pretest": "npm run --silent lint && es-shim-api --bound",
 		"test": "npm run --silent tests-only",
 		"posttest": "aud --production",
 		"tests-only": "npm run --silent test:implementation && npm run --silent test:shim",