Add a Pub source that checks packages out from Git.

utils/pub/system_cache.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * The system-wide cache of installed packages.
  *
  * This cache contains all packages that are downloaded from the internet.
  * Packages that are available locally (e.g. from the SDK) don't use this cache.
  */
 class SystemCache {
   /**
    * The root directory where this package cache is located.
    */
   final String rootDir;
 
   /**
    * Packages which are currently being asynchronously installed to the cache.
    */
   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.
    */
   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));
         });
       });
       return Futures.wait(sources).transform(flatten);
     });
   }
 
   /**
@@ skipping 15 matching lines @@
     var future = exists(path).chain((exists) {
       // TODO(nweiz): better error handling
       if (exists) throw 'Package $id is already installed.';
       return ensureDir(dirname(path));
     }).chain((_) {
       return id.source.install(id, path);
     }).chain((found) {
       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));
     _pendingInstalls[id] = future;
     return future;
   }
 }