After installing jenkins 2.6 locally to test my pipeline scripts, I am unable to use JsonSlurper due to errors about LazyMap not being serializable.

java.io.NotSerializableException: groovy.json.internal.LazyMap

That can't be changed so I looked for a workaround and tried passing the LazyMap into the constructor of a new HashMap:

org.jenkinsci.plugins.scriptsecurity.sandbox.RejectedAccessException: Scripts not permitted to use new java.util.HashMap java.util.Map

So then I found the JsonSlurperClassic class, which should solve the issue by going back to the Groovy 1.x behavior, however then I get:

org.jenkinsci.plugins.scriptsecurity.sandbox.RejectedAccessException: Scripts not permitted to use new groovy.json.JsonSlurperClassic

None of these errors show up in the script approval queue, so I can't whitelist them myself. Please add new JsonSlurperClassic and/or new HashMap to the default whitelist. This will block my upgrade to 2.x otherwise.