[Sass] Add RootNode#to_sass for users to use.

lib/sass/css.rb

@@ -40,7 +40,7 @@ module Sass
         raise Sass::SyntaxError.new(msg, :line => line)
       end
 
-      build_tree.to_sass(0, @options).strip + "\n"
+      build_tree.to_sass(@options).strip + "\n"
     rescue Sass::SyntaxError => err
       err.modify_backtrace(:filename => @options[:filename] || '(css)')
       raise err

lib/sass/tree/node.rb

@@ -190,13 +190,7 @@ module Sass
       # @param opts [{Symbol => Object}] An options hash (see {Sass::CSS#initialize})
       # @return [String] The Sass code corresponding to the node
       def to_sass(tabs = 0, opts = {})
-        result = ''
+        raise NotImplementedError.new("All static-node subclasses of Sass::Tree::Node must override #to_sass.")
-
-        children.each do |child|
-          result << "#{'  ' * tabs}#{child.to_sass(0, opts)}\n"
-        end
-
-        result.rstrip + "\n"
       end
 
       protected

lib/sass/tree/root_node.rb

@@ -44,6 +44,14 @@ module Sass
         super
       end
 
+      # Converts a node to Sass code that will generate it.
+      #
+      # @param opts [{Symbol => Object}] An options hash (see {Sass::CSS#initialize})
+      # @return [String] The Sass code corresponding to the node
+      def to_sass(opts = {})
+        children.map {|child| child.to_sass(0, opts) + "\n"}.join.rstrip + "\n"
+      end
+
       protected
 
       # Destructively converts this static Sass node into a static CSS node,