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

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 28 matching lines @@
   }
 
   void visitTopLevelProduction(TopLevelProduction node) {
     heading('TopLevelProduction', node);
   }
 
   void visitDirective(Directive node) {
     heading('Directive', node);
   }
 
+  void visitCalcTerm(CalcTerm node) {
+    heading('CalcTerm', node);
+    output.depth++;
+    super.visitCalcTerm(node);
+    output.depth--;
+  }
+
   void visitCssComment(CssComment node) {
     heading('Comment', node);
     output.depth++;
     output.writeValue('comment value', node.comment);
     output.depth--;
   }
 
   void visitCommentDefinition(CommentDefinition node) {
     heading('CommentDefinition (CDO/CDC)', node);
     output.depth++;
@@ skipping 490 matching lines @@
   }
 
   void visitPaddingExpression(PaddingExpression node) {
     heading('Dart Style PaddingExpression', node);
   }
 
   void visitWidthExpression(WidthExpression node) {
     heading('Dart Style WidthExpression', node);
   }
 }