Add missing dep (#355)

Author: grouma

webdev/pubspec.yaml

@@ -11,6 +11,7 @@ environment:
 
 dependencies:
   args: ^1.5.0
+  async: ^2.2.0
   build_daemon: '>=0.5.0 <0.7.0'
   built_value: ^6.3.0
   crypto: ^2.0.6