Add a Pub source that checks packages out from Git.

utils/pub/packages_dir.dart

Index: utils/pub/packages_dir.dart
diff --git a/utils/pub/packages_dir.dart b/utils/pub/packages_dir.dart
index 084aeff2fed769958b5ee557dd2e91655f4a6e5b..04f8bd9a8208210a5cc7f6779988e309c722fd08 100644
--- a/utils/pub/packages_dir.dart
+++ b/utils/pub/packages_dir.dart
@@ -80,7 +80,7 @@ class PackagesDir {
             '"${id.source.name}".';
         });
       }
-    }).chain((_) => Package.load(packageDir));
+    }).chain((_) => Package.load(packageDir, cache.sources));
 
     future.then((pkg) => _loadedPackages[id] = pkg);
     always(future, () => _pendingInstalls.remove(id));