Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/barback/src/transformer/declaring_transform.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_transform;
+
+import 'dart:async';
+
+import '../asset/asset_id.dart';
+import 'declaring_aggregate_transform.dart';
+import 'transform_logger.dart';
+
+/// Creates a new [DeclaringTransform] wrapping an
+/// [AggregateDeclaringTransform].
+///
+/// Although barback internally works in terms of
+/// [DeclaringAggregateTransformer]s, most transformers only work on individual
+/// primary inputs in isolation. We want to allow those transformers to
+/// implement the more user-friendly [DeclaringTransformer] interface which
+/// takes the more user-friendly [DeclaringTransform] object. This method wraps
+/// the more general [DeclaringAggregateTransform] to return a
+/// [DeclaringTransform] instead.
+Future<DeclaringTransform> newDeclaringTransform(
+    DeclaringAggregateTransform aggregate) {
+  // A wrapped [Transformer] will assign each primary input a unique transform
+  // key, so we can safely get the first asset emitted. We don't want to wait
+  // for the stream to close, since that requires barback to prove that no more
+  // new assets will be generated.
+  return aggregate.primaryIds.first.then((primaryId) => 
+      new DeclaringTransform._(aggregate, primaryId));
+}
+
+/// A transform for [DeclaringTransformer]s that allows them to declare the ids
+/// of the outputs they'll generate without generating the concrete bodies of
+/// those outputs.
+class DeclaringTransform {
+  /// The underlying aggregate transform.
+  final DeclaringAggregateTransform _aggregate;
+
+  final AssetId primaryId;
+
+  /// A logger so that the [Transformer] can report build details.
+  TransformLogger get logger => _aggregate.logger;
+
+  DeclaringTransform._(this._aggregate, this.primaryId);
+
+  /// Stores [id] as the id of an output that will be created by this
+  /// transformation when it's run.
+  ///
+  /// A transformation can declare as many assets as it wants. If
+  /// [DeclaringTransformer.declareOutputs] declareds a given asset id for a
+  /// given input, [Transformer.apply] should emit the corresponding asset as
+  /// well.
+  void declareOutput(AssetId id) => _aggregate.declareOutput(id);
+
+  /// Consume the primary input so that it doesn't get processed by future
+  /// phases or emitted once processing has finished.
+  ///
+  /// Normally the primary input will automatically be forwarded unless the
+  /// transformer overwrites it by emitting an input with the same id. This
+  /// allows the transformer to tell barback not to forward the primary input
+  /// even if it's not overwritten.
+  void consumePrimary() => _aggregate.consumePrimary(primaryId);
+}