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

lib/src/tree_printer.dart

Index: lib/src/tree_printer.dart
diff --git a/lib/src/tree_printer.dart b/lib/src/tree_printer.dart
index 030a868bcdd604b77877c92a9eb05be02f9fb644..9b0a6c255f4bc2109af69e6d03d75235434b3868 100644
--- a/lib/src/tree_printer.dart
+++ b/lib/src/tree_printer.dart
@@ -46,6 +46,13 @@ class _TreePrinter extends Visitor {
     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++;