pkg/csslib: fix analyzer issue

pkg/csslib/lib/src/tree_printer.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of csslib.visitor;
 
 // TODO(terry): Enable class for debug only; when conditional imports enabled.
 
 /** Helper function to dump the CSS AST. */
 String treeToDebugString(StyleSheet styleSheet, [bool useSpan = false]) {
@@ skipping 154 matching lines @@
     visitDeclarationGroup(node.declarations);
     output.depth--;
   }
 
   /**
    * Added optional newLine for handling @include at top-level vs/ inside of
    * a declaration group.
    */
   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) {
     heading('IncludeMixinAtDeclaration ${node.include.name}', node);
     output.depth++;
     visitIncludeDirective(node.include);
     output.depth--;
   }
 
   void visitExtendDeclaration(ExtendDeclaration node) {
@@ skipping 362 matching lines @@
   }
 
   void visitPaddingExpression(PaddingExpression node) {
     heading('Dart Style PaddingExpression', node);
   }
 
   void visitWidthExpression(WidthExpression node) {
     heading('Dart Style WidthExpression', node);
   }
 }