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

sdk/lib/core/expando.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 dart.core;
 
 /**
  * An [Expando] allows adding new properties to objects.
  */
 class Expando<T> {
 
   /**
    * The name of the this [Expando] as passed to the constructor. If
-   * no name was passed to the constructor, the name is [null].
+   * no name was passed to the constructor, the name is [:null:].
    */
   final String name;
 
   /**
    * Creates a new [Expando]. The optional name is only used for
    * debugging purposes and creating two different [Expando]s with the
    * same name yields two [Expando]s that work on different properties
    * of the objects they are used on.
    */
   external Expando([String name]);
 
   /**
    * Expando toString method override.
    */
   String toString() => "Expando:$name";
 
   /**
    * Gets the value of this [Expando]'s property on the given
    * object. If the object hasn't been expanded, the method returns
-   * [null].
+   * [:null:].
    */
   external T operator [](Object object);
 
   /**
    * Sets the value of this [Expando]'s property on the given
    * object. Properties can effectively be removed again by setting
    * their value to null.
    */
   external void operator []=(Object object, T value);
 
 }