Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/barback/src/transformer/declaring_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.declaring_transformer;
+
+import 'dart:async';
+
+import 'declaring_transform.dart';
+
+/// An interface for [Transformer]s that can cheaply figure out which assets
+/// they'll emit without doing the work of actually creating those assets.
+///
+/// If a transformer implements this interface, that allows barback to perform
+/// optimizations to make the asset graph work more smoothly.
+abstract class DeclaringTransformer {
+  /// Declare which assets would be emitted for the primary input id specified
+  /// by [transform].
+  ///
+  /// This works a little like [Transformer.apply], with two main differences.
+  /// First, instead of having access to the primary input's contents, it only
+  /// has access to its id. Second, instead of emitting [Asset]s, it just emits
+  /// [AssetId]s through [transform.addOutputId].
+  ///
+  /// If this does asynchronous work, it should return a [Future] that completes
+  /// once it's finished.
+  declareOutputs(DeclaringTransform transform);
+}