I think I know how to add the dependency. presumably like this...

DependencyHandler deps = project.getDependencies();
        deps.add( EP, "com.google.guava:guava" + LATEST );
        deps.add( EP, "com.google.errorprone:error_prone_core" + LATEST );
        deps.add( COMPILE, "com.google.errorprone:error_prone_annotations" + LATEST );

but what scope is a plugin added as?