Add missing constraint (#1167)

Author: grouma

pkgs/test/pubspec.yaml

@@ -11,6 +11,7 @@ dependencies:
   async: ^2.0.0
   boolean_selector: ">=1.0.0 <3.0.0"
   coverage: ^0.13.4
+  http: ^0.12.0
   http_multi_server: ^2.0.0
   io: ^0.3.0
   js: ^0.6.0