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

pkg/unittest/lib/src/test_case.dart

 // Copyright (c) 2013, 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 unittest;
 
 /**
  * Represents the state for an individual unit test.
  *
  * Create by calling [test] or [solo_test].
@@ skipping 19 matching lines @@
    * to wait for before the test completes.
    */
   int _callbackFunctionsOutstanding = 0;
 
   String _message = '';
   /** Error or failure message. */
   String get message => _message;
 
   String _result;
   /**
-   * One of [PASS], [FAIL], [ERROR], or [null] if the test hasn't run yet.
+   * One of [PASS], [FAIL], [ERROR], or [:null:] if the test hasn't run yet.
    */
   String get result => _result;
 
   /** Returns whether this test case passed. */
   bool get passed => _result == PASS;
 
   StackTrace _stackTrace;
-  /** Stack trace associated with this test, or [null] if it succeeded. */
+  /** Stack trace associated with this test, or [:null:] if it succeeded. */
   StackTrace get stackTrace => _stackTrace;
 
   /** The group (or groups) under which this test is running. */
   final String currentGroup;
 
   DateTime _startTime;
   DateTime get startTime => _startTime;
 
   Duration _runningTime;
   Duration get runningTime => _runningTime;
@@ skipping 122 matching lines @@
   }
 
   void _markCallbackComplete() {
     if (--_callbackFunctionsOutstanding == 0 && !isComplete) {
       pass();
     }
   }
 
   String toString() => _result != null ? "$description: $result" : description;
 }