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

lib/src/css_printer.dart

Index: lib/src/css_printer.dart
diff --git a/lib/src/css_printer.dart b/lib/src/css_printer.dart
index 125b5aefb41e634931c0be24265c2f4c75d8f58b..a62ca47755562f04dae48ea7d665568efafe5de1 100644
--- a/lib/src/css_printer.dart
+++ b/lib/src/css_printer.dart
@@ -38,6 +38,12 @@ class CssPrinter extends Visitor {
   //              flag for obfuscation.
   bool get _isTesting => !prettyPrint;
 
+  void visitCalcTerm(CalcTerm node) {
+    emit('${node.text}(');
+    node.expr.visit(this);
+    emit(')');
+  }
+
   void visitCssComment(CssComment node) {
     emit('/* ${node.comment} */');
   }