Remove descendants warning while executing tests.

activesupport/lib/active_support/descendants_tracker.rb

@@ -4,9 +4,12 @@ module ActiveSupport
   # This module provides an internal implementation to track descendants
   # which is faster than iterating through ObjectSpace.
   module DescendantsTracker
-    mattr_accessor :descendants
     @@descendants = Hash.new { |h, k| h[k] = [] }
 
+    def self.descendants
+      @@descendants
+    end
+
     def self.clear
       @@descendants.each do |klass, descendants|
         if ActiveSupport::Dependencies.autoloaded?(klass)

activesupport/test/descendants_tracker_test.rb

@@ -70,6 +70,6 @@ class DescendantsTrackerTest < Test::Unit::TestCase
     yield
   ensure
     ActiveSupport::Dependencies.autoloaded_constants = old_autoloaded
-    ActiveSupport::DescendantsTracker.descendants = old_descendants
+    ActiveSupport::DescendantsTracker.descendants.replace(old_descendants)
   end
 end