Start sketching out a buildAll() method.

pkg/barback/lib/src/asset_cascade.dart

Index: pkg/barback/lib/src/asset_cascade.dart
diff --git a/pkg/barback/lib/src/asset_cascade.dart b/pkg/barback/lib/src/asset_cascade.dart
index 802c8fd6f953a8671848d08eab27a65604490f40..f23d02bb9d66a9f8d3129ae96f426253c212d77e 100644
--- a/pkg/barback/lib/src/asset_cascade.dart
+++ b/pkg/barback/lib/src/asset_cascade.dart
@@ -10,6 +10,7 @@ import 'dart:collection';
 import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
+import 'asset_set.dart';
 import 'build_result.dart';
 import 'cancelable_future.dart';
 import 'errors.dart';
@@ -82,6 +83,9 @@ class AssetCascade {
   /// last began.
   var _newChanges = false;
 
+  /// Returns all currently-available output assets from this cascade.
+  AssetSet get availableOutputs => _phases.last.availableOutputs;
+
   /// Creates a new [AssetCascade].
   ///
   /// It loads source assets within [package] using [provider] and then uses