diff --git a/.gitignore b/.gitignore
index aca49089..8013bb17 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,6 +1,7 @@
 .DS_Store
 ._*
 /src/.phutil_module_cache
+/src/extensions/*
 /docs/
 /.divinercache/
 /externals/includes/*
diff --git a/scripts/lib/PhutilLibraryMapBuilder.php b/scripts/lib/PhutilLibraryMapBuilder.php
index 3b0f31eb..861a9352 100755
--- a/scripts/lib/PhutilLibraryMapBuilder.php
+++ b/scripts/lib/PhutilLibraryMapBuilder.php
@@ -388,13 +388,6 @@ final class PhutilLibraryMapBuilder {
 
     $map = array();
     foreach ($files as $file => $hash) {
-      if (basename($file) == '__init__.php') {
-        // TODO: Remove this once we kill __init__.php. This just makes the
-        // script run faster until we do, so testing and development is less
-        // annoying.
-        continue;
-      }
-
       $file = Filesystem::readablePath($file, $root);
       $file = ltrim($file, '/');
 
@@ -404,6 +397,11 @@ final class PhutilLibraryMapBuilder {
         continue;
       }
 
+      if (dirname($file) == 'extensions') {
+        // Ignore files in the extensions/ directory.
+        continue;
+      }
+
       // We include also filename in the hash to handle cases when the file is
       // moved without modifying its content.
       $map[$file] = md5($hash.$file);
diff --git a/src/extensions/README b/src/extensions/README
new file mode 100644
index 00000000..5329621c
--- /dev/null
+++ b/src/extensions/README
@@ -0,0 +1,2 @@
+You can add `.php` files to this directory to load classes and functions defined
+in them at runtime.