From 88d4df437ef35ebe1f8118aeaa18b5a13d3d73b9 Mon Sep 17 00:00:00 2001
From: Henry Zhu <hi@henryzoo.com>
Date: Thu, 6 Apr 2017 11:59:41 -0400
Subject: [PATCH] use a Set

---
 experimental/babel-preset-env/src/index.js                | 5 ++++-
 experimental/babel-preset-env/src/use-built-ins-plugin.js | 4 ++--
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/experimental/babel-preset-env/src/index.js b/experimental/babel-preset-env/src/index.js
index 5400dad9ff..b1c8842094 100644
--- a/experimental/babel-preset-env/src/index.js
+++ b/experimental/babel-preset-env/src/index.js
@@ -243,7 +243,10 @@ export default function buildPreset(context, opts = {}) {
   );
 
   if (useBuiltIns === true) {
-    plugins.push([addUsedBuiltInsPlugin, { polyfills, regenerator, debug }]);
+    plugins.push([
+      addUsedBuiltInsPlugin,
+      { polyfills: new Set(polyfills), regenerator, debug },
+    ]);
   } else if (useBuiltIns === "entry") {
     plugins.push([useBuiltInsEntryPlugin, { polyfills, regenerator }]);
   }
diff --git a/experimental/babel-preset-env/src/use-built-ins-plugin.js b/experimental/babel-preset-env/src/use-built-ins-plugin.js
index 6861264fc4..c4f7a3b7fa 100644
--- a/experimental/babel-preset-env/src/use-built-ins-plugin.js
+++ b/experimental/babel-preset-env/src/use-built-ins-plugin.js
@@ -38,12 +38,12 @@ export default function({ types: t }) {
   function addUnsupported(path, polyfills, builtIn, builtIns) {
     if (Array.isArray(builtIn)) {
       for (const i of builtIn) {
-        if (polyfills.indexOf(i) !== -1) {
+        if (polyfills.has(i)) {
           addImport(path, `core-js/modules/${i}`, builtIns);
         }
       }
     } else {
-      if (polyfills.indexOf(builtIn) !== -1) {
+      if (polyfills.has(builtIn)) {
         addImport(path, `core-js/modules/${builtIn}`, builtIns);
       }
     }