Fix two checked mode bugs introduced by 0.13.0.

lib/src/tree.dart

 // Copyright (c) 2012, 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;
 
 /////////////////////////////////////////////////////////////////////////
 // CSS specific types:
 /////////////////////////////////////////////////////////////////////////
 
@@ skipping 678 matching lines @@
     return new MixinDefinition(name, cloneDefinedArgs, varArgs, span);
   }
 
   visit(VisitorBase visitor) => visitor.visitMixinDefinition(this);
 }
 
 /** Support a Sass @mixin. See http://sass-lang.com for description. */
 class MixinRulesetDirective extends MixinDefinition {
   final List<TreeNode> rulesets;
 
-  MixinRulesetDirective(String name, List<VarDefinition> args,
+  MixinRulesetDirective(String name, List<TreeNode> args,
       bool varArgs, this.rulesets, SourceSpan span)
       : super(name, args, varArgs, span);
 
   MixinRulesetDirective clone() {
     var clonedArgs = <VarDefinition>[];
     for (var arg in definedArgs) {
       clonedArgs.add(arg.clone());
     }
     var clonedRulesets = <TreeNode>[];
     for (var ruleset in rulesets) {
@@ skipping 722 matching lines @@
 
   PaddingExpression._merge(
       PaddingExpression x, PaddingExpression y, SourceSpan span)
       : super(DartStyleExpression.paddingStyle, span,
           new BoxEdge.merge(x.box, y.box));
 
   PaddingExpression clone() => new PaddingExpression(span,
       top: box.top, right: box.right, bottom: box.bottom, left: box.left);
   visit(VisitorBase visitor) => visitor.visitPaddingExpression(this);
 }