[Sass] Move detection of nodes only allowed at the root into Tree::Node.

lib/sass/engine.rb

@@ -298,17 +298,6 @@ END
     end
 
     def validate_and_append_child(parent, child, line, root)
-      unless root
-        case child
-        when Tree::MixinDefNode
-          raise SyntaxError.new("Mixins may only be defined at the root of a document.",
-            :line => line.index)
-        when Tree::ImportNode
-          raise SyntaxError.new("Import directives may only be used at the root of a document.",
-            :line => line.index)
-        end
-      end
-
       case child
       when Array
         child.each {|c| validate_and_append_child(parent, c, line, root)}

lib/sass/tree/node.rb

@@ -293,7 +293,8 @@ module Sass
       # Returns an error message if the given child node is invalid,
       # and false otherwise.
       #
-      # By default, all child nodes are valid.
+      # By default, all child nodes except those only allowed at root level
+      # ({Tree::MixinDefNode}, {Tree::ImportNode}) are valid.
       # This is expected to be overriden by subclasses
       # for which some children are invalid.
       #
@@ -301,7 +302,12 @@ module Sass
       # @return [Boolean, String] Whether or not the child node is valid,
       #   as well as the error message to display if it is invalid
       def invalid_child?(child)
-        false
+        case child
+        when Tree::MixinDefNode
+          "Mixins may only be defined at the root of a document."
+        when Tree::ImportNode
+          "Import directives may only be used at the root of a document."
+        end
       end
     end
   end

lib/sass/tree/root_node.rb

@@ -70,6 +70,14 @@ module Sass
         return "" if result.empty?
         return result + "\n"
       end
+
+      # Returns false, because all nodes are allowed at the root of the document
+      # (properties are detected elsewhere post-mixin-resolution).
+      #
+      # @see Node#invalid_child?
+      def invalid_child?(child)
+        false
+      end
     end
   end
 end