[Sass] Sass::Plugin is not expected to be thread safe in development …

…mode at this point.

lib/sass/plugin/staleness_checker.rb

@@ -1,9 +1,15 @@
 module Sass
   module Plugin
     class StalenessChecker
-      def initialize
+      @dependencies_cache = {}
+
+      class << self
+        attr_accessor :dependencies_cache
+      end
+
+      def initialize(dependencies = self.class.dependencies_cache)
+        @dependencies                = dependencies
         @mtimes, @dependencies_stale = {}, {}
-        @dependencies = Thread.current[:_sass_file_dependencies] ||= {}
       end
 
       def stylesheet_needs_update?(css_file, template_file)

test/sass/plugin_test.rb

@@ -357,6 +357,7 @@ def touch(*args)
   end
 
   def reset_mtimes
+    Sass::Plugin::StalenessChecker.dependencies_cache = {}
     atime = Time.now
     mtime = Time.now - 5
     Dir["{#{template_loc},#{tempfile_loc}}/**/*.{css,sass,scss}"].each {|f| File.utime(atime, mtime, f)}