add cache list command to pub

utils/pub/git_source.dart

Index: utils/pub/git_source.dart
===================================================================
--- utils/pub/git_source.dart	(revision 19836)
+++ utils/pub/git_source.dart	(working copy)
@@ -116,6 +116,10 @@
     });
   }
 
+  Future<List<Package>> getCachedPackages() {
+   return new Future.immediate([]);

[Bob Nystrom, 2013/03/12 22:12:03]

Throw UnimplementedError().

[nweiz, 2013/03/12 22:58:48]

I'm not sure throwing UnimplementedError is necessary, since the parent class
will already throw. I would add a TODO to implement this, though.

[keertip, 2013/03/13 15:55:00]

Done.

+  }
+  
   /// Ensure that the canonical clone of the repository referred to by [id] (the
   /// one in `<system cache>/git/cache`) exists and is up-to-date. Returns a
   /// future that completes once this is finished and throws an exception if it