Fix the usage instructions for matcher and mock libraries.

pkg/unittest/lib/mock.dart

Index: pkg/unittest/lib/mock.dart
===================================================================
--- pkg/unittest/lib/mock.dart	(revision 22522)
+++ pkg/unittest/lib/mock.dart	(working copy)
@@ -11,13 +11,17 @@
  * file.
  *
  *     dependencies:
- *       mock: any
+ *       unittest: any
  *
  * Then run `pub install`.
  *
- * For more information, see the
- * [mock package on pub.dartlang.org](http://pub.dartlang.org/packages/mock).
+ * Import this into your Dart code with:
  *
+ *     import 'package:unittest/mock.dart';
+ *
+ * For more information, see the [unittest package on pub.dartlang.org]
+ * (http://pub.dartlang.org/packages/unittest).
+ *
  * ## Using ##
  *
  * To create a mock objects for some class T, create a new class using: