Add a Pub source that checks packages out from Git.

utils/pub/packages_dir.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 "packages" directory for an application or library.
  *
  * This directory contains symlinks to all packages used by an app. These links
  * point either to the [SystemCache] or to some other location on the local
  * filesystem.
@@ skipping 62 matching lines @@
         return cache.install(id).chain(
             (pkg) => createSymlink(pkg.dir, packageDir));
       } else {
         return id.source.install(id, packageDir).transform((found) {
           if (found) return null;
           // TODO(nweiz): More robust error-handling.
           throw 'Package ${id.fullName} not found in source '
             '"${id.source.name}".';
         });
       }
-    }).chain((_) => Package.load(packageDir));
+    }).chain((_) => Package.load(packageDir, cache.sources));
 
     future.then((pkg) => _loadedPackages[id] = pkg);
     always(future, () => _pendingInstalls.remove(id));
     _pendingInstalls[id] = future;
 
     return future;
   }
 
   /**
    * Installs the package identified by [id] and all its transitive
@@ skipping 16 matching lines @@
 
   /**
    * Installs all dependencies of [owner] to the "packages" directory. Returns a
    * [Future] that completes when all dependencies are installed.
    */
   Future installDependencies() {
     return Futures.wait(owner.dependencies.map(installTransitively)).
       transform((_) => null);
   }
 }