diff --git a/build/tasks/build-task.coffee b/build/tasks/build-task.coffee
index 0b71a7a36..a6f74bffd 100644
--- a/build/tasks/build-task.coffee
+++ b/build/tasks/build-task.coffee
@@ -87,6 +87,10 @@ module.exports = (grunt) ->
       path.join('snippets', 'node_modules', 'pegjs')
       path.join('snippets', 'node_modules', '.bin', 'pegjs')
 
+      # These aren't needed since WeakMap is built-in
+      path.join('emissary', 'node_modules', 'es6-weak-map')
+      path.join('property-accessors', 'node_modules', 'es6-weak-map')
+
       '.DS_Store'
       '.jshintrc'
       '.npmignore'