Handle synchronous asset load errors in barback.

pkg/barback/lib/src/graph/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.graph.asset_cascade;
 
 import 'dart:async';
 
 import '../asset/asset.dart';
 import '../asset/asset_id.dart';
 import '../asset/asset_node.dart';
 import '../asset/asset_set.dart';
 import '../errors.dart';
 import '../log.dart';
 import '../transformer/transformer.dart';
+import '../utils.dart';
 import '../utils/cancelable_future.dart';
 import 'node_status.dart';
 import 'node_streams.dart';
 import 'package_graph.dart';
 import 'phase.dart';
 
 /// The asset cascade for an individual package.
 ///
 /// This keeps track of which [Transformer]s are applied to which assets, and
 /// re-runs those transformers when their dependencies change. The transformed
@@ skipping 105 matching lines @@
         controller.setDirty();
       } else {
         _sourceControllerMap[id] = new AssetNodeController(id);
         _phases.first.addInput(_sourceControllerMap[id].node);
       }
 
       // If this source was already loading, cancel the old load, since it may
       // return out-of-date contents for the asset.
       if (_loadingSources.containsKey(id)) _loadingSources[id].cancel();
 
-      _loadingSources[id] =
-          new CancelableFuture<Asset>(graph.provider.getAsset(id));
+      _loadingSources[id] = new CancelableFuture<Asset>(
+          syncFuture(() => graph.provider.getAsset(id)));
       _loadingSources[id].whenComplete(() {
         _loadingSources.remove(id);
       }).then((asset) {
         var controller = _sourceControllerMap[id].setAvailable(asset);
       }).catchError((error, stack) {
         reportError(new AssetLoadException(id, error, stack));
 
         // TODO(nweiz): propagate error information through asset nodes.
         _sourceControllerMap.remove(id).setRemoved();
       });
@@ skipping 60 matching lines @@
     _streams.onLogPool.add(phase.onLog);
     if (_phaseStatusSubscription != null) _phaseStatusSubscription.cancel();
     _phaseStatusSubscription =
         phase.onStatusChange.listen(_streams.changeStatus);
 
     _phases.add(phase);
   }
 
   String toString() => "cascade for $package";
 }