Added beginning support for calc however, the expression is not fully parsed into the AST.

lib/visitor.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.
 
 library csslib.visitor;
 
 import 'package:source_span/source_span.dart';
 import 'parser.dart';
 
 part 'src/css_printer.dart';
 part 'src/tree.dart';
 part 'src/tree_base.dart';
 part 'src/tree_printer.dart';
 
 abstract class VisitorBase {
+  visitCalcTerm(CalcTerm node);
   visitCssComment(CssComment node);
   visitCommentDefinition(CommentDefinition node);
   visitStyleSheet(StyleSheet node);
   visitNoOp(NoOp node);
   visitTopLevelProduction(TopLevelProduction node);
   visitDirective(Directive node);
   visitMediaExpression(MediaExpression node);
   visitMediaQuery(MediaQuery node);
   visitMediaDirective(MediaDirective node);
   visitHostDirective(HostDirective node);
@@ skipping 99 matching lines @@
   visitStyleSheet(StyleSheet ss) {
     _visitNodeList(ss.topLevels);
   }
 
   visitNoOp(NoOp node) {}
 
   visitTopLevelProduction(TopLevelProduction node) {}
 
   visitDirective(Directive node) {}
 
+  visitCalcTerm(CalcTerm node) {
+    visitLiteralTerm(node);
+    visitLiteralTerm(node.expr);
+  }
+
   visitCssComment(CssComment node) {}
 
   visitCommentDefinition(CommentDefinition node) {}
 
   visitMediaExpression(MediaExpression node) {
     visitExpressions(node.exprs);
   }
 
   visitMediaQuery(MediaQuery node) {
     for (var mediaExpr in node.expressions) {
@@ skipping 301 matching lines @@
   visitPaddingExpression(PaddingExpression node) {
     // TODO(terry): TBD
     throw new UnimplementedError();
   }
 
   visitWidthExpression(WidthExpression node) {
     // TODO(terry): TBD
     throw new UnimplementedError();
   }
 }