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

pkg/csslib/lib/src/analyzer.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.
 
 part of csslib.parser;
 
 
 // TODO(terry): Add optimizing phase to remove duplicated selectors in the same
 //              selector group (e.g., .btn, .btn { color: red; }).  Also, look
 //              at simplifying selectors expressions too (much harder).
@@ skipping 157 matching lines @@
  *
  * Nested rules is a terse mechanism to describe CSS inheritance.  The analyzer
  * will flatten and expand the nested rules to it's flatten strucure.  Using the
  * all parent [RuleSets] (selector expressions) and applying each nested
  * [RuleSet] to the list of [Selectors] in a [SelectorGroup].
  *
  * Then result is a style sheet where all nested rules have been flatten and
  * expanded.
  */
 class ExpandNestedSelectors extends Visitor {
-  /** Parent [RuleSet] if a nested rule otherwise [null]. */
+  /** Parent [RuleSet] if a nested rule otherwise [:null:]. */
   RuleSet _parentRuleSet;
 
   /** Top-most rule if nested rules. */
   SelectorGroup _topLevelSelectorGroup;
 
   /** SelectorGroup at each nesting level. */
   SelectorGroup _nestedSelectorGroup;
 
   /** Declaration (sans the nested selectors). */
   DeclarationGroup _flatDeclarationGroup;
@@ skipping 819 matching lines @@
             isLastNone = false;
           }
         } else {
           isLastNone = simpleSeq.isCombinatorNone;
         }
       }
     }
     super.visitSelectorGroup(node);
   }
 }