Modularize master branch.

_baseAssignIn.js

@@ -0,0 +1,17 @@
+import copyObject from './_copyObject.js';
+import keysIn from './keysIn.js';
+
+/**
+ * The base implementation of `_.assignIn` without support for multiple sources
+ * or `customizer` functions.
+ *
+ * @private
+ * @param {Object} object The destination object.
+ * @param {Object} source The source object.
+ * @returns {Object} Returns `object`.
+ */
+function baseAssignIn(object, source) {
+  return object && copyObject(source, keysIn(source), object);
+}
+
+export default baseAssignIn;