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

lib/visitor.dart

Index: lib/visitor.dart
diff --git a/lib/visitor.dart b/lib/visitor.dart
index 593e43a5517912433c6ca0c58bffc39709fd2bf8..b6babbdf5c80ab7dee56352a8af895fd6abc179d 100644
--- a/lib/visitor.dart
+++ b/lib/visitor.dart
@@ -13,6 +13,7 @@ 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);
@@ -132,6 +133,11 @@ class Visitor implements VisitorBase {
 
   visitDirective(Directive node) {}
 
+  visitCalcTerm(CalcTerm node) {
+    visitLiteralTerm(node);
+    visitLiteralTerm(node.expr);
+  }
+
   visitCssComment(CssComment node) {}
 
   visitCommentDefinition(CommentDefinition node) {}