Give cached sources control over their cache directories.

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('package.dart');
 #import('source_registry.dart');
 
 /**
@@ skipping 52 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 = join(rootDir, id.source.name, id.source.packageName(id));
+    var sourceDir = join(rootDir, id.source.name);
+    var path = id.source.systemCacheDirectory(id, sourceDir);
     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, sources);
     });
 
     always(future, () => _pendingInstalls.remove(id));
     _pendingInstalls[id] = future;
     return future;
   }
 }