Asset load failures in Barback should produce AssetNotFoundExceptions.

pkg/barback/test/package_graph/errors_test.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.test.package_graph.source_test;
 
 import 'package:barback/src/utils.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../utils.dart';
@@ skipping 128 matching lines @@
 
     setAssetError("app|foo.txt");
     updateSources(["app|foo.txt"]);
     expectNoAsset("app|foo.txt");
     buildShouldFail([
       isAssetLoadException("app|foo.txt", isMockLoadException("app|foo.txt"))
     ]);
   });
 
   test("an asset isn't passed through a transformer with an error", () {
-    initGraph(["app|foo.txt",], {"app": [[new BadTransformer([])]]});
+    initGraph(["app|foo.txt"], {"app": [[new BadTransformer([])]]});
 
     updateSources(["app|foo.txt"]);
     expectNoAsset("app|foo.txt");
     buildShouldFail([isTransformerException(equals(BadTransformer.ERROR))]);
   });
+
+  test("a transformer can catch an error loading a secondary input", () {
+    initGraph(["app|foo.txt"], {"app": [
+      [new CatchAssetNotFoundTransformer(".txt", "app|nothing")]
+    ]});
+
+    updateSources(["app|foo.txt"]);
+    expectAsset("app|foo.txt", "failed to load app|nothing");
+    buildShouldSucceed();
+  });
 }