add cache list command to pub

utils/pub/source.dart

Index: utils/pub/source.dart
===================================================================
--- utils/pub/source.dart	(revision 19836)
+++ utils/pub/source.dart	(working copy)
@@ -215,6 +215,12 @@
   ///
   /// By default, this just returns [id].
   Future<PackageId> resolveId(PackageId id) => new Future.immediate(id);
+  
+  /// Returns a list of [Package] that have been cached in in Pub's global cache

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

Returns the [Package]s that have been installed in the system cache.

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

Done.

+  /// directory.
+  Future<List<Package>> getCachedPackages() {
+    if (shouldCache) throw "Source $name must implement this.";
+  }
 
   /// Returns the source's name.
   String toString() => name;