Support loading transformer plugins from pub.

sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart

Index: sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart b/sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart
new file mode 100644
index 0000000000000000000000000000000000000000..9e1d7b43c48d532585b95a56dc1c59416705af38
--- /dev/null
+++ b/sdk/lib/_internal/pub/lib/src/barback/pub_package_provider.dart
@@ -0,0 +1,27 @@
+// Copyright (c) 2013, 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 pub.pub_package_provider;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+import 'package:path/path.dart' as path;
+
+import '../package_graph.dart';
+
+/// An implementation of barback's [PackageProvider] interface so that barback
+/// can find assets within pub packages.
+class PubPackageProvider implements PackageProvider {
+  final PackageGraph _graph;
+
+  PubPackageProvider(this._graph);
+
+  Iterable<String> get packages => _graph.packages.keys;
+
+  Future<Asset> getAsset(AssetId id) {
+    var file = path.join(_graph.packages[id.package].dir, id.path);
+    return new Future.value(new Asset.fromPath(id, file));
+  }
+}