Bump to v4.0.0.

internal/cloneSymbol.js

@@ -0,0 +1,18 @@
+import _Symbol from './_Symbol';
+
+/** Used to convert symbols to primitives and strings. */
+var symbolProto = _Symbol ? _Symbol.prototype : undefined,
+    symbolValueOf = _Symbol ? symbolProto.valueOf : undefined;
+
+/**
+ * Creates a clone of the `symbol` object.
+ *
+ * @private
+ * @param {Object} symbol The symbol object to clone.
+ * @returns {Object} Returns the cloned symbol object.
+ */
+function cloneSymbol(symbol) {
+  return _Symbol ? Object(symbolValueOf.call(symbol)) : {};
+}
+
+export default cloneSymbol;