[Refactor] avoid top-level return, because babel and webpack are broken

Closes inspect-js/node-deep-equal#79.
Closes inspect-js/node-deep-equal#78.
Closes es-shims/Promise.allSettled#7.
Closes airbnb/js-shims#12.

Relates to:
 - inspect-js/is-map#5
 - inspect-js/is-map#4
 - inspect-js/is-map#3
 - inspect-js/is-set#5
 - inspect-js/is-set#4
 - storybookjs/storybook#9154.

index.js

@@ -3,27 +3,27 @@
 var $WeakMap = typeof WeakMap === 'function' && WeakMap.prototype ? WeakMap : null;
 var $WeakSet = typeof WeakSet === 'function' && WeakSet.prototype ? WeakSet : null;
 
+var exported;
+
 if (!$WeakMap) {
 	// eslint-disable-next-line no-unused-vars
-	module.exports = function isWeakMap(x) {
+	exported = function isWeakMap(x) {
 		// `WeakMap` is not present in this environment.
 		return false;
 	};
-	return;
 }
 
 var $mapHas = $WeakMap ? $WeakMap.prototype.has : null;
 var $setHas = $WeakSet ? $WeakSet.prototype.has : null;
-if (!$mapHas) {
+if (!exported && !$mapHas) {
 	// eslint-disable-next-line no-unused-vars
-	module.exports = function isWeakMap(x) {
+	exported = function isWeakMap(x) {
 		// `WeakMap` does not have a `has` method
 		return false;
 	};
-	return;
 }
 
-module.exports = function isWeakMap(x) {
+module.exports = exported || function isWeakMap(x) {
 	if (!x || typeof x !== 'object') {
 		return false;
 	}