Support loading transformer plugins from pub.

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

Index: sdk/lib/_internal/pub/lib/src/package_graph.dart
diff --git a/sdk/lib/_internal/pub/lib/src/package_graph.dart b/sdk/lib/_internal/pub/lib/src/package_graph.dart
new file mode 100644
index 0000000000000000000000000000000000000000..687b7202ace05cbed9dd68757723106d6a4213fb
--- /dev/null
+++ b/sdk/lib/_internal/pub/lib/src/package_graph.dart
@@ -0,0 +1,21 @@
+// 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.package_graph;
+
+import 'entrypoint.dart';
+import 'package.dart';
+
+/// A holistic view of the entire transitive dependency graph for an entrypoint.
+///
+/// A package graph can be loaded using [Entrypoint.loadPackageGraph].
+class PackageGraph {
+  /// The entrypoint.
+  final Entrypoint entrypoint;
+
+  /// All transitive dependencies of the entrypoint (including itself).
+  final Map<String, Package> packages;
+
+  PackageGraph(this.entrypoint, this.packages);
+}