Make the Git source install to the system cache.

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.
 
 #library('system_cache');
 
 #import('io.dart');
 #import('package.dart');
 #import('source.dart');
 #import('source_registry.dart');
@@ skipping 64 matching lines @@
    * == false` to the system cache.
    */
   Future<Package> install(PackageId id) {
     if (!id.source.shouldCache) {
       throw new IllegalArgumentException("Package $id is not cacheable.");
     }
 
     var pending = _pendingInstalls[id];
     if (pending != null) return pending;
 
-    var path = id.source.systemCacheDirectory(id);
-    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 not found.';
-      return Package.load(path, sources);
-    });
-
+    var future = id.source.installToSystemCache(id);
     always(future, () => _pendingInstalls.remove(id));
     _pendingInstalls[id] = future;
     return future;
   }
 }