Prepare for replacing PackageBundle with PackageBundleReader.

There are clients in google3 that import package:analyzer/src/summary/idl.dart
to use PackageBundle. We cannot make changes to idl.dart, so this
CL introduces a new file that we can change, and in the binary format CL
we will replace the exported PackageBundle class with a new class for
the binary format.

Change-Id: Iee89c784a2812bcdeefaef1c122ac8d8533665fe
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/174080
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/summary2/package_bundle_reader.dart

@@ -0,0 +1,5 @@
+// Copyright (c) 2020, 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.
+
+export 'package:analyzer/src/summary/idl.dart' show PackageBundle;