pkg/csslib: fix analyzer issue

R=terry@google.com

Review URL: https://codereview.chromium.org//64993002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/csslib@30062 260f80e4-7a28-3924-810f-c04153c831b5
diff --git a/lib/src/tree_printer.dart b/lib/src/tree_printer.dart
index 8c39476..3d381bd 100644
--- a/lib/src/tree_printer.dart
+++ b/lib/src/tree_printer.dart
@@ -172,7 +172,8 @@
    */
   void visitIncludeDirective(IncludeDirective node) {
     heading('IncludeDirective ${node.name}', node);
-    output.writeNodeList('parameters', node.args);
+    var flattened = node.args.expand((e) => e).toList();
+    output.writeNodeList('parameters', flattened);
   }
 
   void visitIncludeMixinAtDeclaration(IncludeMixinAtDeclaration node) {