Make the Git source install to the system cache.

utils/pub/source.dart

Index: utils/pub/source.dart
diff --git a/utils/pub/source.dart b/utils/pub/source.dart
index 21144ce3ea8c56405cce0d50bc4db5993d20c866..236aa9a5eb2897c8637bbfa3ccf42805cb957c61 100644
--- a/utils/pub/source.dart
+++ b/utils/pub/source.dart
@@ -95,15 +95,42 @@ class Source {
    *
    * [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));