[Sass] Leave FileNodes unresolved until perform-time.

This works better with precompilation,
since a file only needs to be re-compiled if the file itself has been modified.

lib/sass/engine.rb

@@ -437,7 +437,7 @@ END
 
         next Tree::DirectiveNode.new("@import url(#{filename})") if filename =~ /\.css$/
 
-        Tree::FileNode.new(filename, Sass::Files.tree_for(filename, @options).children)
+        Tree::FileNode.new(filename)
       end.flatten
     end
   end

lib/sass/tree/file_node.rb

@@ -1,9 +1,9 @@
 module Sass
   module Tree
     class FileNode < Node
-      def initialize(filename, children)
+      def initialize(filename)
         @filename = filename
-        self.children = children
+        super()
       end
 
       def to_s(*args)
@@ -16,6 +16,7 @@ module Sass
       protected
 
       def perform!(environment)
+        self.children = Sass::Files.tree_for(filename, @options).children
         self.children = perform_children(environment)
       rescue Sass::SyntaxError => e
         e.add_backtrace_entry(@filename)