Add a Pub source that checks packages out from Git.

utils/pub/system_cache.dart

Index: utils/pub/system_cache.dart
diff --git a/utils/pub/system_cache.dart b/utils/pub/system_cache.dart
index d4f41d3df63ccfe3d19c4da15dcb5084b397dcc7..8f4b5bd110e8df906b9de60d2647a12487d55463 100644
--- a/utils/pub/system_cache.dart
+++ b/utils/pub/system_cache.dart
@@ -20,11 +20,17 @@ class SystemCache {
   final Map<PackageId, Future<Package>> _pendingInstalls;
 
   /**
+   * The sources from which to install packages.
+   */
+  final SourceRegistry sources;
+
+  /**
    * Creates a new package cache which is backed by the given directory on the
    * user's file system.
    */
   SystemCache(this.rootDir)
-  : _pendingInstalls = new Map<PackageId, Future<Package>>();
+  : _pendingInstalls = new Map<PackageId, Future<Package>>(),
+    sources = new SourceRegistry();
 
   /**
    * Loads all of the package ids in the cache and returns them.
@@ -32,7 +38,7 @@ class SystemCache {
   Future<List<PackageId>> listAll() {
     return listDir(rootDir).chain((paths) {
       final sources = paths.map((path) {
-        final source = Source.fromName(basename(path));
+        final source = sources[basename(path)];
         return listDir(path).transform((subpaths) {
           return subpaths.map((subpath) =>
             new PackageId(basename(subpath), source));
@@ -68,7 +74,7 @@ class SystemCache {
       if (!found) {
         throw 'Package ${id.fullName} not found in source "${id.source.name}".';
       }
-      return Package.load(path);
+      return Package.load(path, sources);
     });
 
     always(future, () => _pendingInstalls.remove(id));