Move internal modules to “internal” folder.

.internal/addMapEntry.js

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