Add a new --extra-lib option to apidoc that allows us to specify an explicit .dart file that should…

pkg/unittest/lib/unittest.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.
 
 /**
  * A library for writing dart unit tests.
  *
  * ## Installing ##
  *
  * Use [pub][] to install this package. Add the following to your `pubspec.yaml`
@@ skipping 155 matching lines @@
  */
 library unittest;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:isolate';
 import 'dart:math' show max;
 import 'matcher.dart';
 export 'matcher.dart';
 
-// TODO(amouravski): We should not need to import mock here, but it's necessary
-// to enable dartdoc on the mock library, as it's not picked up normally.
-import 'mock.dart';
-
 part 'src/config.dart';
 part 'src/test_case.dart';
 
 Configuration _config;
 
 /**
  * [Configuration] used by the unittest library. Note that if a 
  * configuration has not been set, calling this getter will create
  * a default configuration.
  */
@@ skipping 746 matching lines @@
         }
         sb.write(location);
         sb.write(' ');
         sb.write(position);
         sb.write('\n');
       }
     }
   }
   return sb.toString();
 }