Now in sprockets-rails 3 app.assets is set only in case when assets.compile option is enabled (https://github.com/rails/sprockets-rails/blob/master/lib/sprockets/railtie.rb#L142).

By default in Rails this option is set to false in production environment: https://github.com/rails/rails/blob/master/railties/lib/rails/generators/rails/app/templates/config/environments/production.rb.tt#L27

This leads us to the bug when some gem calls Rails.application.assets.resolve("asset.js") in production environment and the NoMethodError is raised.

@josh does it sounds like a bug?