Refactor Source.

sdk/lib/_internal/pub/lib/src/system_cache.dart

Index: sdk/lib/_internal/pub/lib/src/system_cache.dart
diff --git a/sdk/lib/_internal/pub/lib/src/system_cache.dart b/sdk/lib/_internal/pub/lib/src/system_cache.dart
index 9cfca43046c3c820b8b6f75b105a8f162e1e4711..4c48fbad859612912fca43d9125b477a4f2cbb4d 100644
--- a/sdk/lib/_internal/pub/lib/src/system_cache.dart
+++ b/sdk/lib/_internal/pub/lib/src/system_cache.dart
@@ -12,6 +12,7 @@ import 'io.dart';
 import 'io.dart' as io show createTempDir;
 import 'log.dart' as log;
 import 'package.dart';
+import 'source/cached.dart';
 import 'source/git.dart';
 import 'source/hosted.dart';
 import 'source/path.dart';
@@ -29,17 +30,12 @@ class SystemCache {
 
   String get tempDir => path.join(rootDir, '_temp');
 
-  /// Packages which are currently being asynchronously downloaded to the cache.
-  final Map<PackageId, Future<Package>> _pendingDownloads;
-
   /// The sources from which to get packages.
-  final SourceRegistry sources;
+  final sources = new SourceRegistry();
 
   /// Creates a new package cache which is backed by the given directory on the
   /// user's file system.
-  SystemCache(this.rootDir)
-  : _pendingDownloads = new Map<PackageId, Future<Package>>(),
-    sources = new SourceRegistry();
+  SystemCache(this.rootDir);
 
   /// Creates a system cache and registers the standard set of sources. If
   /// [isOffline] is `true`, then the offline hosted source will be used.
@@ -70,40 +66,13 @@ class SystemCache {
   Future<bool> contains(PackageId id) {
     var source = sources[id.source];
 
-    if (!source.shouldCache) {
+    if (source is! CachedSource) {
       throw new ArgumentError("Package $id is not cacheable.");
     }
 
     return source.isInSystemCache(id);
   }
 
-  /// Ensures that the package identified by [id] is downloaded to the cache,
-  /// loads it, and returns it.
-  ///
-  /// It is an error to try downloading a package from a source with
-  /// `shouldCache == false`.
-  ///
-  /// If [force] is `true`, then the package is downloaded even if it already
-  /// exists in the cache. The previous one will be deleted.
-  Future<Package> download(PackageId id, {bool force}) {
-    var source = sources[id.source];
-
-    if (!source.shouldCache) {
-      throw new ArgumentError("Package $id is not cacheable.");
-    }
-
-    var pending = _pendingDownloads[id];
-    if (pending != null) return pending;
-
-    var future = source.downloadToSystemCache(id, force: force)
-        .whenComplete(() {
-      _pendingDownloads.remove(id);
-    });
-
-    _pendingDownloads[id] = future;
-    return future;
-  }
-
   /// Create a new temporary directory within the system cache. The system
   /// cache maintains its own temporary directory that it uses to stage
   /// packages into while downloading. It uses this instead of the OS's system