Add mojo_services pub dependency from sky sdk

BUG=
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1224623007.

sdk/pubspec.yaml

@@ -2,6 +2,7 @@ author: Chromium Authors <sky-dev@googlegroups.com>
 dependencies:
   mojo: '>=0.0.1 <1.0.0'
   mojom: '>=0.0.4 <1.0.0'
+  mojo_services: '>=0.0.4 <1.0.0'
   vector_math: '>=1.4.3 <2.0.0'
 description: Dart files to support executing inside Sky.
 homepage: https://github.com/domokit/mojo/tree/master/sky