Start sketching out a buildAll() method.

pkg/barback/lib/src/asset_cascade.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.asset_cascade;
 
 import 'dart:async';
 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';
 import 'package_graph.dart';
 import 'phase.dart';
 import 'transformer.dart';
 import 'utils.dart';
 
 /// The asset cascade for an individual package.
 ///
@@ skipping 52 matching lines @@
 
   /// A future that completes when the currently running build process finishes.
   ///
   /// If no build it in progress, is `null`.
   Future _processDone;
 
   /// Whether any source assets have been updated or removed since processing
   /// 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
   /// [transformerPhases] to generate output files from them.
   //TODO(rnystrom): Better way of specifying transformers and their ordering.
   AssetCascade(this.graph, this.package,
       Iterable<Iterable<Transformer>> transformerPhases) {
     // Flatten the phases to a list so we can traverse backwards to wire up
     // each phase to its next.
     var phases = transformerPhases.toList();
@@ skipping 146 matching lines @@
 
         // Otherwise, everything is done.
         return;
       }
 
       // Process that phase and then loop onto the next.
       return future.then((_) => _process());
     });
   }
 }