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

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 26 matching lines @@
 }
 
 class Negation extends TreeNode {
   Negation(SourceSpan span) : super(span);
   Negation clone() => new Negation(span);
   visit(VisitorBase visitor) => visitor.visitNegation(this);
 
   String get name => 'not';
 }
 
+// calc(...)
+// TODO(terry): Hack to handle calc however the expressions should be fully
+//              parsed and in the AST.
+class CalcTerm extends LiteralTerm {
+  final LiteralTerm expr;
+
+  CalcTerm(var value, String t, this.expr, SourceSpan span)
+      : super(value, t, span);
+
+  CalcTerm clone() => new CalcTerm(value, text, expr.clone(), span);
+  visit(VisitorBase visitor) => visitor.visitCalcTerm(this);
+
+  String toString() => "$text($expr)";
+}
+
 // /*  ....   */
 class CssComment extends TreeNode {
   final String comment;
 
   CssComment(this.comment, SourceSpan span) : super(span);
   CssComment clone() => new CssComment(comment, span);
   visit(VisitorBase visitor) => visitor.visitCssComment(this);
 }
 
 // CDO/CDC (Comment Definition Open <!-- and Comment Definition Close -->).
@@ skipping 1362 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);
 }