Remove barback from the repo.

pkg/barback/lib/src/package_provider.dart

-// 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);
-}