Start sketching out a buildAll() method.

pkg/barback/lib/src/package_graph.dart

Index: pkg/barback/lib/src/package_graph.dart
diff --git a/pkg/barback/lib/src/package_graph.dart b/pkg/barback/lib/src/package_graph.dart
index 639deea6f49300cb47f6e4729e07d787a45a93e6..b0e3135bbe846fee2816a9ce2717fa95fb746b64 100644
--- a/pkg/barback/lib/src/package_graph.dart
+++ b/pkg/barback/lib/src/package_graph.dart
@@ -9,6 +9,7 @@ import 'dart:async';
 import 'asset_cascade.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
+import 'asset_set.dart';
 import 'build_result.dart';
 import 'errors.dart';
 import 'package_provider.dart';
@@ -92,6 +93,31 @@ class PackageGraph {
     return new Future.value(null);
   }
 
+  /// Gets all output assets.
+  ///
+  /// If a build is currently in progress, waits until it completes. The
+  /// returned future will complete with an error if the build is not
+  /// successful.
+  Future<AssetSet> getAllAssets() {
+    if (_cascadeResults.values.contains(null)) {
+      // A build is still ongoing, so wait for it to complete and try again.
+      return results.first.then((_) => getAllAssets());
+    }
+
+    // If the build completed with an error, complete the future with it.
+    var errors = unionAll(
+        _cascadeResults.values.map((result) => result.errors));
+    if (errors.isNotEmpty) {
+      return new Future.error(BarbackException.aggregate(errors));
+    }
+
+    // Otherwise, return all of the final output assets.
+    var assets = unionAll(_cascades.values.map(
+        (cascade) => cascade.availableOutputs.toSet()));
+
+    return new Future.value(new AssetSet.from(assets));
+  }
+
   /// Adds [sources] to the graph's known set of source assets.
   ///
   /// Begins applying any transforms that can consume any of the sources. If a