Gradle allows the creation of dynamic properties. If a property does not yet exist, it will be created. An example:
In your example snippet you try to set the httpPort on the jettyStop task. As the httpPort property does not exist for the jettyStop task (see dsl reference at http://gradle.org/docs/current/dsl/org.gradle.api.plugins.jetty.JettyStop.html) groovy dynamically creates this object. As we changed this default behaviour to avoid accidently created properties, the deprecation warning is written.