use proper notation for true, false, and null in doc comments

pkg/csslib/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_maps/span.dart' show SourceFile, Span, FileSpan;
 
@@ skipping 984 matching lines @@
    *
    * Two tag name selectors div and span equivalent to:
    *
    *    div {
    *      width: 20px;
    *    }
    *    div span {
    *      color: red;
    *    }
    *
-   * Return [null] if no selector or [SelectorGroup] if a selector was parsed.
+   * Return [:null:] if no selector or [SelectorGroup] if a selector was parsed.
    */
   SelectorGroup _nestedSelector() {
     Messages oldMessages = messages;
     _createMessages();
 
     var markedData = _mark;
 
     // Look a head do we have a nested selector instead of a declaration?
     SelectorGroup selGroup = processSelectorGroup();
 
@@ skipping 1648 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();
 }