Start sketching out a buildAll() method.

pkg/barback/lib/src/barback.dart

Index: pkg/barback/lib/src/barback.dart
diff --git a/pkg/barback/lib/src/barback.dart b/pkg/barback/lib/src/barback.dart
index 3dcb23e66d166996a44b27bd55c3606041e794aa..cb097fa8428a802ba079a01b2c02a94d01204efd 100644
--- a/pkg/barback/lib/src/barback.dart
+++ b/pkg/barback/lib/src/barback.dart
@@ -8,6 +8,7 @@ import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
+import 'asset_set.dart';
 import 'build_result.dart';
 import 'errors.dart';
 import 'package_graph.dart';
@@ -88,4 +89,11 @@ class Barback {
   /// Removes [removed] from the graph's known set of source assets.
   void removeSources(Iterable<AssetId> removed) =>
       _graph.removeSources(removed);
+
+  /// Gets all output assets.
+  ///
+  /// If a build is currently in progress, waits until it completes. The
+  /// returned future will complete with a [BarbackException] if the build is
+  /// not successful.
+  Future<AssetSet> getAllAssets() => _graph.getAllAssets();
 }