[Fix] ensure that zloirock/core-js#144 doesn't cause false positives.

Also add `npm run test:corejs`.

Relates to #4.

getCollectionsForEach.js

@@ -1,6 +1,23 @@
 module.exports = function () {
-	var mapForEach = typeof Map === 'function' ? Map.prototype.forEach : null;
-	var setForEach = typeof Set === 'function' ? Set.prototype.forEach : null;
+	var mapForEach = (function () {
+		if (typeof Map !== 'function') { return null; }
+		try {
+			Map.prototype.forEach.call({}, function () {});
+		} catch (e) {
+			return Map.prototype.forEach;
+		}
+		return null;
+	}());
+
+	var setForEach = (function () {
+		if (typeof Set !== 'function') { return null; }
+		try {
+			Set.prototype.forEach.call({}, function () {});
+		} catch (e) {
+			return Set.prototype.forEach;
+		}
+		return null;
+	}());
 
 	return { Map: mapForEach, Set: setForEach };
 };

package.json

@@ -7,9 +7,10 @@
 	"main": "index.js",
 	"scripts": {
 		"test": "npm run lint && npm run tests-only && npm run security",
-		"tests-only": "npm run test:native && npm run test:shimmed",
+		"tests-only": "npm run test:native && npm run test:shimmed && npm run test:corejs",
 		"test:native": "node test/native.js",
 		"test:shimmed": "node test/shimmed.js",
+		"test:corejs": "node test/corejs.js",
 		"coverage": "covert test/native.js",
 		"coverage-quiet": "covert test/native.js --quiet",
 		"lint": "npm run jscs && npm run eslint",
@@ -58,7 +59,8 @@
 		"@ljharb/eslint-config": "^1.6.0",
 		"nsp": "^2.1.0",
 		"replace": "^0.3.0",
-		"es6-shim": "^0.33.13"
+		"es6-shim": "^0.33.13",
+		"core-js": "^1.2.6"
 	},
 	"testling": {
 		"files": "test/native.js",

test/corejs.js

@@ -0,0 +1,5 @@
+'use strict';
+
+require('core-js');
+
+require('./native');