Trying to make use of extension methods but haven't managed to get it work.
In example, outside of pipeline this fails:

String.metaClass.toTitleCase = { -> toLowerCase().tokenize(" ")*.capitalize().join(" ") }

returning "spread not yet supported", but below code in post stage of pipeline (making html email) works fine:

entry.User = "${fields[1]}".replace(".", " ").trim().tokenize(" ")*.capitalize().join(" ")

which may mean that spread actually works.
 
replacing extension method with: 

String.metaClass.toTitleCase = {
  def words = phrase.toLowerCase().tokenize(" ")
  words.collect{ -> capitalize() }
  String result = words.join(" ")
  return result;
}

gives other error: "No signature of method: java.lang.String.toTitleCase() is applicable for argument types: () values: []"

I think there is some error around handling extension methods here.

abayer could you check if its not like the bug https://issues.jenkins-ci.org/browse/JENKINS-47193 where classes outside pipeline were not being parsed?