Give cached sources control over their cache directories.

Review URL: https://chromiumcodereview.appspot.com//10577009

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@8832 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/source.dart

@@ -35,15 +35,21 @@ class Source {
    * to true if the package was found in the source and false if it wasn't. For
    * all other error conditions, it should complete with an exception.
    *
-   * If [shouldCache] is true, [path] will be a path to this source's
-   * subdirectory of the [PackageCache]'s cache directory. If [shouldCache] is
-   * false, [path] will be a path to the application's "packages" directory.
-   *
    * [path] is guaranteed not to exist, and its parent directory is guaranteed
    * to exist.
    */
   abstract Future<bool> install(PackageId id, String path);
 
+  /**
+   * Returns the directory in the system cache that the package identified by
+   * [id] should be installed to. [parent] is this source's subdirectory in the
+   * system cache directory.
+   *
+   * This doesn't need to be implemented if [shouldCache] is false.
+   */
+  String systemCacheDirectory(PackageId id, String parent) =>
+    join(parent, packageName(id));
+
   /**
    * When a [Pubspec] is parsed, it reads in the description for each
    * dependency. It is up to the dependency's [Source] to determine how that

lib/src/system_cache.dart

@@ -70,7 +70,8 @@ class SystemCache {
     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.';