Make PhaseOutput emit a new AssetNode whenever it changes outputs.

pkg/barback/test/transformer/check_content_and_rename.dart

Index: pkg/barback/test/transformer/check_content_and_rename.dart
diff --git a/pkg/barback/test/transformer/check_content_and_rename.dart b/pkg/barback/test/transformer/check_content_and_rename.dart
new file mode 100644
index 0000000000000000000000000000000000000000..a923911311637245b7feb7830eded9d2e660e8da
--- /dev/null
+++ b/pkg/barback/test/transformer/check_content_and_rename.dart
@@ -0,0 +1,35 @@
+// 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.check_content_and_rename;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+import 'mock.dart';
+
+/// A transformer that checks the extension and content of an asset, then
+/// produces a new asset with a new extension and new content.
+class CheckContentAndRenameTransformer extends MockTransformer {
+  final String oldExtension;
+  final String oldContent;
+  final String newExtension;
+  final String newContent;
+
+  CheckContentAndRenameTransformer(this.oldExtension, this.oldContent,
+      this.newExtension, this.newContent);
+
+  Future<bool> doIsPrimary(Asset asset) {
+    if (asset.id.extension != '.$oldExtension') return new Future.value(false);
+    return asset.readAsString().then((value) => value == oldContent);
+  }
+
+  Future doApply(Transform transform) {
+    return getPrimary(transform).then((input) {
+      transform.addOutput(new Asset.fromString(
+          input.id.changeExtension('.$newExtension'), newContent));
+    });
+  }
+}