Make the Git source install to the system cache.

utils/pub/source.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('source');
 
 #import('io.dart');
 #import('package.dart');
 #import('pubspec.dart');
 #import('system_cache.dart');
@@ skipping 77 matching lines @@
   }
 
   /**
    * Installs the package identified by [id] to [path]. Returns a [Future] that
    * completes when the installation was finished. The [Future] should resolve
    * 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.
    *
    * [path] is guaranteed not to exist, and its parent directory is guaranteed
    * to exist.
+   *
+   * This doesn't need to be implemented if [installToSystemCache] is
+   * implemented.
    */
-  abstract Future<bool> install(PackageId id, String path);
+  Future<bool> install(PackageId id, String path) {
+    throw "Either install or installToSystemCache must be implemented for "
+      "source $name."
+  }
+
+  /**
+   * Installs the package identified by [id] to the system cache. This is only
+   * called for sources with [shouldCache] set to true.
+   *
+   * By default, this uses [systemCacheDirectory] and [install].
+   */
+  Future<Package> installToSystemCache(PackageId id) {
+    var path = systemCacheDirectory(id);
+    return exists(path).chain((exists) {
+      // TODO(nweiz): better error handling
+      if (exists) throw 'Package $id is already installed.';
+      return ensureDir(dirname(path));
+    }).chain((_) {
+      return install(id, path);
+    }).chain((found) {
+      if (!found) throw 'Package $id not found.';
+      return Package.load(path, systemCache.sources);
+    });
+  }
 
   /**
    * Returns the directory in the system cache that the package identified by
    * [id] should be installed to. This should return a path to a subdirectory of
    * [systemCacheRoot].
    *
-   * This doesn't need to be implemented if [shouldCache] is false.
+   * This doesn't need to be implemented if [shouldCache] is false, or if
+   * [installToSystemCache] is implemented.
    */
   String systemCacheDirectory(PackageId id) =>
     join(systemCacheRoot, packageName(id.description));
 
   /**
    * 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
    * should be interpreted. This will be called during parsing to validate that
    * the given [description] is well-formed according to this source. It should
    * return if the description is valid, or throw a [FormatException] if not.
@@ skipping 14 matching lines @@
    * Returns whether or not [description1] describes the same package as
    * [description2] for this source. This method should be light-weight. It
    * doesn't need to validate that either package exists.
    *
    * By default, this assumes both descriptions are strings and compares them
    * for equality.
    */
   bool descriptionsEqual(description1, description2) =>
     description1 == description2;
 }