From 6f5e47eff07225f4a95de23b13494729ab42aa26 Mon Sep 17 00:00:00 2001
From: Henry Zhu <hi@henryzoo.com>
Date: Sat, 16 Apr 2016 18:27:55 -0400
Subject: [PATCH] Check if `t.is` is already defined (specifically accounting
 for the isImmutable method in validators

---
 packages/babel-types/src/index.js | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/packages/babel-types/src/index.js b/packages/babel-types/src/index.js
index 911fba5cdb..5a4b581112 100644
--- a/packages/babel-types/src/index.js
+++ b/packages/babel-types/src/index.js
@@ -12,9 +12,12 @@ let t = exports;
  */
 
 function registerType(type: string) {
-  let is = t[`is${type}`] = function (node, opts) {
-    return t.is(type, node, opts);
-  };
+  let is = t[`is${type}`];
+  if (!is) {
+    is = t[`is${type}`] = function (node, opts) {
+      return t.is(type, node, opts);
+    };
+  }
 
   t[`assert${type}`] = function (node, opts) {
     opts = opts || {};