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

lib/parser.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.
 
 library csslib.parser;
 
 import 'dart:math' as math;
 
 import 'package:source_span/source_span.dart';
 
@@ skipping 2170 matching lines @@
           _error('Expecting a positive number', _makeSpan(start));
         }
 
         _eat(TokenKind.RBRACK);
 
         return new ItemTerm(term.value, term.text, _makeSpan(start));
       case TokenKind.IDENTIFIER:
         var nameValue = identifier(); // Snarf up the ident we'll remap, maybe.
 
         if (!ieFilter && _maybeEat(TokenKind.LPAREN)) {
+          var calc = processCalc(nameValue);
+          if (calc != null) return calc;
           // FUNCTION
           return processFunction(nameValue);
         }
         if (ieFilter) {
           if (_maybeEat(TokenKind.COLON) &&
               nameValue.name.toLowerCase() == 'progid') {
             // IE filter:progid:
             return processIEFilter(start);
           } else {
             // Handle filter:<name> where name is any filter e.g., alpha, chroma,
@@ skipping 234 matching lines @@
                 startAfterProgidColon.start.offset, _peekToken.start);
             return new LiteralTerm(tok.text, tok.text, tok.span);
           }
           break;
         default:
           _eat(_peek());
       }
     }
   }
 
+  //  TODO(terry): Hack to gobble up the calc expression as a string looking
+  //               for the matching RPAREN the expression is not parsed into the
+  //               AST.
+  //
+  //  grammar should be:
+  //
+  //    <calc()> = calc( <calc-sum> )
+  //    <calc-sum> = <calc-product> [ [ '+' | '-' ] <calc-product> ]*
+  //    <calc-product> = <calc-value> [ '*' <calc-value> | '/' <number> ]*
+  //    <calc-value> = <number> | <dimension> | <percentage> | ( <calc-sum> )
+  //
+  String processCalcExpression() {
+    var inString = tokenizer._inString;
+    tokenizer._inString = false;
+
+    // Gobble up everything until we hit our stop token.
+    var stringValue = new StringBuffer();
+    var left = 1;
+    var matchingParens = false;
+    while (_peek() != TokenKind.END_OF_FILE && !matchingParens) {
+      var token = _peek();
+      if (token == TokenKind.LPAREN)
+        left++;
+      else if (token == TokenKind.RPAREN)
+        left--;
+
+      matchingParens = left == 0;
+      if (!matchingParens) stringValue.write(_next().text);
+    }
+
+    if (!matchingParens) {
+      _error("problem parsing function expected ), ", _peekToken.span);
+    }
+
+    tokenizer._inString = inString;
+
+    return stringValue.toString();
+  }
+
+  CalcTerm processCalc(Identifier func) {
+    var start = _peekToken.span;
+
+    var name = func.name;
+    if (name == 'calc') {
+      // TODO(terry): Implement expression parsing properly.
+      String expression = processCalcExpression();
+      var calcExpr = new LiteralTerm(expression, expression, _makeSpan(start));
+
+      if (!_maybeEat(TokenKind.RPAREN)) {
+        _error("problem parsing function expected ), ", _peekToken.span);
+      }
+
+      return new CalcTerm(name, name, calcExpr, _makeSpan(start));
+    }
+
+    return null;
+  }
+
   //  Function grammar:
   //
   //  function:     IDENT '(' expr ')'
   //
   processFunction(Identifier func) {
     var start = _peekToken.span;
 
     var name = func.name;
 
     switch (name) {
       case 'url':
         // URI term sucks up everything inside of quotes(' or ") or between parens
         var urlParam = processQuotedString(true);
 
         // TODO(terry): Better error messge and checking for mismatched quotes.
         if (_peek() == TokenKind.END_OF_FILE) {
           _error("problem parsing URI", _peekToken.span);
         }
 
         if (_peek() == TokenKind.RPAREN) {
           _next();
         }
 
         return new UriTerm(urlParam, _makeSpan(start));
-      case 'calc':
-        // TODO(terry): Implement expression handling...
-        break;
       case 'var':
         // TODO(terry): Consider handling var in IE specific filter/progid.  This
         //              will require parsing entire IE specific syntax e.g.,
         //              param = value or progid:com_id, etc. for example:
         //
         //    var-blur: Blur(Add = 0, Direction = 225, Strength = 10);
         //    var-gradient: progid:DXImageTransform.Microsoft.gradient"
         //      (GradientType=0,StartColorStr='#9d8b83', EndColorStr='#847670');
         var expr = processExpr();
         if (!_maybeEat(TokenKind.RPAREN)) {
@@ skipping 207 matching lines @@
 
     if (replace != null && result == null) {
       result = new StringBuffer(text.substring(0, i));
     }
 
     if (result != null) result.write(replace != null ? replace : text[i]);
   }
 
   return result == null ? text : result.toString();
 }