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

pkg/csslib/lib/src/tokenkind.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): Need to be consistent with tokens either they're ASCII tokens
 //              e.g., ASTERISK or they're CSS e.g., PSEUDO, COMBINATOR_*.
 class TokenKind {
   // Common shared tokens used in TokenizerBase.
@@ skipping 565 matching lines @@
           return entry['value'];
         }
       }
     }
 
     return '<BAD UNIT>';  // Not a unit token.
   }
 
   /**
    * Match color name, case insensitive match and return the associated color
-   * entry from _EXTENDED_COLOR_NAMES list, return [null] if not found.
+   * entry from _EXTENDED_COLOR_NAMES list, return [:null:] if not found.
    */
   static Map matchColorName(String text) {
     var name = text.toLowerCase();
     return _EXTENDED_COLOR_NAMES.
         firstWhere((e) => e['name'] == name, orElse: () => null);
   }
 
   /** Return RGB value as [int] from a color entry in _EXTENDED_COLOR_NAMES. */
   static int colorValue(Map entry) {
     assert(entry != null);
@@ skipping 161 matching lines @@
   static const int EQUALS = 0x3d; // "=".codeUnitAt(0)
   static const int OR = 0x7c; // "|".codeUnitAt(0)
   static const int CARET = 0x5e; // "^".codeUnitAt(0)
   static const int DOLLAR = 0x24; // "\$".codeUnitAt(0)
   static const int LESS = 0x3c; // "<".codeUnitAt(0)
   static const int BANG = 0x21; // "!".codeUnitAt(0)
   static const int MINUS = 0x2d; // "-".codeUnitAt(0)
   static const int BACKSLASH = 0x5c; // "\".codeUnitAt(0)
   static const int AMPERSAND = 0x26; // "&".codeUnitAt(0)
 }