NoSuchMethodError: method from standard scala library. After upgrade of scala version

I have some scala code which I built to jar and I spark-submit it to hadoop cluster. I bumped up version of scala from 2.11 to 2.12. In result I got runtime excetpion:

java.lang.NoSuchMethodError:scala.collection.JavaConversions
$.asScalaSet(Ljava/util/Set;)Lscala/collection/mutable/Set

I think that I replaced everywhere in my code scala version. The problem is that I have no idea how to debug it?

First of all, my question is:
How scala manage its dependencies? Should I change something on remote cluster (I send there jars via spark-submit)? In other words, there is no asScalaSet method that comes from standard scala library. How to check if it is present on cluster? After all, my code is compiled to class files and after this compilation it has nothing in common with scala and can be executed by JVM.