Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/barback/src/transformer/lazy_transformer.dart

+// Copyright (c) 2014, 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 barback.transformer.lazy_transformer;
+
+import 'declaring_transformer.dart';
+
+/// An interface for [Transformer]s that indicates that the transformer's
+/// outputs shouldn't be generated until requested.
+///
+/// The [declareOutputs] method is used to figure out which assets should be
+/// treated as "lazy." Lazy assets will only be forced to be generated if
+/// they're requested by the user or if they're used by a non-declaring
+/// transformer.
+abstract class LazyTransformer extends DeclaringTransformer {}