From 4163d1372a89388f007b47dd6d57a8055ea61907 Mon Sep 17 00:00:00 2001
From: Sebastian McKenzie <sebmck@gmail.com>
Date: Tue, 3 Feb 2015 10:23:47 +1100
Subject: [PATCH] fix istanbul interop - closes #660

---
 lib/6to5/register.js | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/lib/6to5/register.js b/lib/6to5/register.js
index a11008ec26..bd2f60d006 100644
--- a/lib/6to5/register.js
+++ b/lib/6to5/register.js
@@ -72,16 +72,27 @@ var shouldIgnore = function (filename) {
   return (ignoreRegex && ignoreRegex.test(filename)) || (onlyRegex && !onlyRegex.test(filename));
 };
 
-var istanbulLoader = function (m, filename, old) {
+var istanbulMonkey = {};
+
+if (process.env.running_under_istanbul) { // jshint ignore:line
   // we need to monkey patch fs.readFileSync so we can hook into
   // what istanbul gets, it's extremely dirty but it's the only way
   var _readFileSync = fs.readFileSync;
 
-  fs.readFileSync = function () {
-    fs.readFileSync = _readFileSync;
-    return compile(filename);
+  fs.readFileSync = function (filename) {
+    if (istanbulMonkey[filename]) {
+      delete istanbulMonkey[filename];
+      var code = compile(filename);
+      istanbulMonkey[filename] = true;
+      return code;
+    } else {
+      return _readFileSync.apply(this, arguments);
+    }
   };
+}
 
+var istanbulLoader = function (m, filename, old) {
+  istanbulMonkey[filename] = true;
   old(m, filename);
 };