diff --git a/lib/third-party-libs.js.flow b/lib/third-party-libs.js.flow
index 7e6fd06d5d..1aefc40618 100644
--- a/lib/third-party-libs.js.flow
+++ b/lib/third-party-libs.js.flow
@@ -7,3 +7,27 @@ declare module "micromatch" {
     nocase: boolean,
   }): Array<string>;
 }
+
+declare module "resolve" {
+  declare export default {
+    sync: (string, {| basedir: string |}) => string;
+  };
+}
+
+declare module "json5" {
+  declare export default {
+    parse: (string) => mixed,
+  };
+}
+
+declare module "lodash/defaults" {
+  declare export default <T: Object>(T, Object) =>  T;
+}
+
+declare module "lodash/clone" {
+  declare export default <T>(obj: T) => T;
+ }
+
+declare module "lodash/merge" {
+  declare export default <T: Object>(T, Object) => T;
+}
diff --git a/packages/babel-core/src/config/helpers/merge.js b/packages/babel-core/src/config/helpers/merge.js
deleted file mode 100644
index 344f3e181c..0000000000
--- a/packages/babel-core/src/config/helpers/merge.js
+++ /dev/null
@@ -1,21 +0,0 @@
-// @flow
-
-import mergeWith from "lodash/mergeWith";
-
-export default function<T: {}>(dest?: T, src?: T) {
-  if (!dest || !src) return;
-
-  mergeWith(dest, src, function (a, b) {
-    if (b && Array.isArray(a)) {
-      const newArray = b.slice(0);
-
-      for (const item of a) {
-        if (newArray.indexOf(item) < 0) {
-          newArray.push(item);
-        }
-      }
-
-      return newArray;
-    }
-  });
-}
diff --git a/packages/babel-core/src/config/option-manager.js b/packages/babel-core/src/config/option-manager.js
index 456cdc43c1..a5cfc63bbd 100644
--- a/packages/babel-core/src/config/option-manager.js
+++ b/packages/babel-core/src/config/option-manager.js
@@ -4,7 +4,7 @@ import * as context from "../index";
 import Plugin from "./plugin";
 import * as messages from "babel-messages";
 import defaults from "lodash/defaults";
-import merge from "./helpers/merge";
+import merge from "lodash/merge";
 import removed from "./removed";
 import buildConfigChain from "./build-config-chain";
 import path from "path";