class ActiveSupport::DescendantsTracker::WeakSet

:nodoc:
have the same WeakMap semantic than Truffle so we future proof this as much as possible.
JRuby for now doesn't have Class#descendant, but when it will, it will likely

So we use `object_id` as a key and the actual object as a value.
On TruffleRuby `ObjectSpace::WeakMap` keys are strong references.

def initialize
  @map = ObjectSpace::WeakMap.new
end