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

sdk/lib/_internal/compiler/implementation/tree/prettyprint.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 tree;
 
 /**
  * Pretty-prints Node tree in XML-like format.
  *
  * TODO(smok): Add main() to run from command-line to print out tree for given
@@ skipping 217 matching lines @@
   }
 
   visitLiteralDouble(LiteralDouble node) {
     printLiteral(node, "LiteralDouble");
   }
 
   visitLiteralInt(LiteralInt node) {
     printLiteral(node, "LiteralInt");
   }
 
-  /** Returns token string value or [null] if token is [null]. */
+  /** Returns token string value or [:null:] if token is [:null:]. */
   tokenToStringOrNull(Token token) => token == null ? null : token.stringValue;
 
   visitLiteralList(LiteralList node) {
     openNode(node, "LiteralList", {
       "constKeyword" : tokenToStringOrNull(node.constKeyword)
     });
     visitChildNode(node.typeArguments, "typeArguments");
     visitChildNode(node.elements, "elements");
     closeNode();
   }
@@ skipping 226 matching lines @@
   }
 
   visitStringNode(StringNode node) {
     unimplemented('visitNode', node: node);
   }
 
   unimplemented(String message, {Node node}) {
     throw message;
   }
 }