Add the ability for a PackageProvider to declare static packages.

pkg/barback/lib/src/graph/static_asset_cascade.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.graph.static_asset_cascade;
+
+import 'dart:async';
+
+import '../asset/asset_id.dart';
+import '../asset/asset_node.dart';
+import '../asset/asset_set.dart';
+import '../errors.dart';
+import '../log.dart';
+import '../package_provider.dart';
+import 'asset_cascade.dart';
+import 'node_status.dart';
+import 'package_graph.dart';
+
+/// An asset cascade for a static package.
+///
+/// A static package is known to have no transformers and no changes to its
+/// assets. This allows this class to lazily and efficiently provide assets to
+/// the rest of the package graph.
+class StaticAssetCascade implements AssetCascade {
+  final String package;
+
+  final PackageGraph graph;
+
+  /// All sources that have been requested from the provider.
+  final _sources = new Map<AssetId, Future<AssetNode>>();
+
+  StaticAssetCascade(this.graph, this.package);
+
+  Stream<BarbackException> get errors => _errorsController.stream;
+  final _errorsController =
+      new StreamController<BarbackException>.broadcast(sync: true);
+
+  final status = NodeStatus.IDLE;
+
+  final onLog = new StreamController<LogEntry>.broadcast().stream;
+  final onStatusChange = new StreamController<LogEntry>.broadcast().stream;
+
+  Future<AssetSet> get availableOutputs {
+    var provider = graph.provider as StaticPackageProvider;
+    return provider.getAllAssetIds(package).map(provider.getAsset).toList()
+        .then((assets) => new AssetSet.from(assets));
+  }
+
+  Future<AssetNode> getAssetNode(AssetId id) {
+    return _sources.putIfAbsent(id, () {
+      return graph.provider.getAsset(id).then((asset) {
+        return new AssetNodeController.available(asset).node;
+      }).catchError((error, stackTrace) {
+        if (error is! AssetNotFoundException) {
+          reportError(new AssetLoadException(id, error, stackTrace));
+        }
+
+        // TODO(nweiz): propagate error information through asset nodes.
+        return null;
+      });
+    });
+  }
+
+  void updateSources(Iterable<AssetId> sources) =>
+      throw new UnsupportedError("Static package $package can't be explicitly "
+          "provided sources.");
+
+  void removeSources(Iterable<AssetId> sources) =>
+      throw new UnsupportedError("Static package $package can't be explicitly "
+          "provided sources.");
+
+  void updateTransformers(Iterable<Iterable> transformersIterable) =>
+      throw new UnsupportedError("Static package $package can't have "
+          "transformers.");
+
+  void forceAllTransforms() {}
+
+  void reportError(BarbackException error) => _errorsController.add(error);
+
+  String toString() => "static cascade for $package";
+}