Asset load failures in Barback should produce AssetNotFoundExceptions.

pkg/barback/test/transformer/catch_asset_not_found.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.transformer.bad;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+import 'package:barback/src/utils.dart';
+
+import 'mock.dart';
+
+/// A transformer that tries to load a secondary input and catches an
+/// [AssetNotFoundException] if the input doesn't exist.
+class CatchAssetNotFoundTransformer extends MockTransformer {
+  /// The extension of assets this applies to.
+  final String extension;
+
+  /// The id of the secondary input to load.
+  final AssetId input;
+
+  CatchAssetNotFoundTransformer(this.extension, String input)
+      : input = new AssetId.parse(input);
+
+  Future<bool> doIsPrimary(Asset asset) =>
+      new Future.value(asset.id.extension == extension);
+
+  Future doApply(Transform transform) {
+    return transform.getInput(input).then((_) {
+      transform.addOutput(new Asset.fromString(
+          transform.primaryInput.id, "success"));
+    }).catchError((e) {
+      if (e is! AssetNotFoundException) throw e;
+      transform.addOutput(new Asset.fromString(
+          transform.primaryInput.id, "failed to load $input"));
+    });
+  }
+}