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

sdk/lib/_internal/lib/isolate_helper.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 _isolate_helper;
 
 import 'dart:async';
 import 'dart:collection' show Queue, HashMap;
 import 'dart:isolate';
 import 'dart:_js_helper' show
@@ skipping 134 matching lines @@
 
   /**
    * Whether to use web workers when implementing isolates. Set to false for
    * debugging/testing.
    */
   bool get useWorkers => supportsWorkers;
 
   /**
    * Whether to use the web-worker JSON-based message serialization protocol. By
    * default this is only used with web workers. For debugging, you can force
-   * using this protocol by changing this field value to [true].
+   * using this protocol by changing this field value to [:true:].
    */
   bool get needSerialization => useWorkers;
 
   /**
    * Registry of isolates. Isolates must be registered if, and only if, receive
    * ports are alive.  Normally no open receive-ports means that the isolate is
    * dead, but DOM callbacks could resurrect it.
    */
   Map<int, _IsolateContext> isolates;
 
@@ skipping 1146 matching lines @@
       _handle = null;
     } else {
       throw new UnsupportedError("Canceling a timer.");
     }
   }
 
   bool get isActive => _handle != null;
 }
 
 bool hasTimer() => JS('', '#.setTimeout', globalThis) != null;