First pass at build dependency graph for barback.

pkg/barback/lib/src/asset_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.asset_provider;
+
+import 'dart:async';
+
+import 'asset.dart';
+import '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 AssetProvider {
+  /// 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;
+
+  // TODO(rnystrom): Make this async.
+  /// The paths of all available asset files in [package], relative to the
+  /// package's root directory.
+  ///
+  /// You can pass [within], which should be the relative path to a directory
+  /// within the package, to only return the files within that subdirectory.
+  List<AssetId> listAssets(String package, {String within});
+
+  Future<Asset> getAsset(AssetId id);
+}