Add observatory_pub_packages snapshot to third_party

observatory_pub_packages/barback/src/package_provider.dart

Index: observatory_pub_packages/barback/src/package_provider.dart
===================================================================
--- observatory_pub_packages/barback/src/package_provider.dart	(revision 0)
+++ observatory_pub_packages/barback/src/package_provider.dart	(working copy)
@@ -0,0 +1,49 @@
+// 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 barback.package_provider;
+
+import 'dart:async';
+
+import 'asset/asset.dart';
+import 'asset/asset_id.dart';
+
+/// API for locating and accessing packages on disk.
+///
+/// Implemented by pub and provided to barback so that it isn't coupled
+/// directly to pub.
+abstract class PackageProvider {
+  /// The names of all packages that can be provided by this provider.
+  ///
+  /// This is equal to the transitive closure of the entrypoint package
+  /// dependencies.
+  Iterable<String> get packages;
+
+  /// Loads an asset from disk.
+  ///
+  /// This should be re-entrant; it may be called multiple times with the same
+  /// id before the previously returned future has completed.
+  ///
+  /// If no asset with [id] exists, the provider should throw an
+  /// [AssetNotFoundException].
+  Future<Asset> getAsset(AssetId id);
+}
+
+/// A PackageProvider for which some packages are known to be static—that is,
+/// the package has no transformers and its assets won't ever change.
+///
+/// For static packages, rather than telling barback up-front which assets that
+/// package contains via [Barback.updateSources], barback will lazily query the
+/// provider for an asset when it's needed. This is much more efficient.
+abstract class StaticPackageProvider implements PackageProvider {
+  /// The names of all static packages provided by this provider.
+  ///
+  /// This must be disjoint from [packages].
+  Iterable<String> get staticPackages;
+
+  /// Returns all ids of assets in [package].
+  ///
+  /// This is used for [Barback.getAllAssets].
+  Stream<AssetId> getAllAssetIds(String package);
+}