Move internal modules to “internal” folder.

.internal/addSetEntry.js

@@ -0,0 +1,15 @@
+/**
+ * Adds `value` to `set`.
+ *
+ * @private
+ * @param {Object} set The set to modify.
+ * @param {*} value The value to add.
+ * @returns {Object} Returns `set`.
+ */
+function addSetEntry(set, value) {
+  // Don't return `set.add` because it's not chainable in IE 11.
+  set.add(value);
+  return set;
+}
+
+export default addSetEntry;