Add missing constraint (#1167)

diff --git a/pkgs/test/pubspec.yaml b/pkgs/test/pubspec.yaml
index a3ba98a..7d58b1f 100644
--- a/pkgs/test/pubspec.yaml
+++ b/pkgs/test/pubspec.yaml
@@ -11,6 +11,7 @@
   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