Add the ability for a PackageProvider to declare static packages.

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);
+}