Follow the First Law of Extend (see #324).

Some tests still fail, since they rely on the old behavior which violated this
law.

lib/sass/selector/comma_sequence.rb

@@ -49,7 +49,15 @@ def resolve_parent_refs(super_cseq)
       # @return [CommaSequence] A copy of this selector,
       #   with extensions made according to `extends`
       def do_extend(extends)
-        CommaSequence.new(members.map {|seq| seq.do_extend(extends)}.flatten)
+        CommaSequence.new(members.map do |seq|
+            extended = seq.do_extend(extends)
+            # First Law of Extend: the result of extending a selector should
+            # always contain the base selector.
+            #
+            # See https://github.com/nex3/sass/issues/324.
+            extended.unshift seq unless seq.has_placeholder? || extended.include?(seq)
+            extended
+          end.flatten)
       end
 
       # Returns a string representation of the sequence.