Make PhaseOutput emit a new AssetNode whenever it changes outputs.

pkg/barback/test/package_graph/group_test.dart

Index: pkg/barback/test/package_graph/group_test.dart
diff --git a/pkg/barback/test/package_graph/group_test.dart b/pkg/barback/test/package_graph/group_test.dart
index c81b3b6997c56a935bf2a3980267e6eee5621090..f42de356e4661b3b292afc32d0855bd405ea20ee 100644
--- a/pkg/barback/test/package_graph/group_test.dart
+++ b/pkg/barback/test/package_graph/group_test.dart
@@ -4,6 +4,8 @@
 
 library barback.test.package_graph.group_test;
 
+import 'dart:async';
+
 import 'package:barback/barback.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
@@ -313,6 +315,31 @@ main() {
       expectAsset("app|foo.a", "foo.a");
       buildShouldSucceed();
     });
+
+    test("doesn't pass-through an asset that ceases to be forwarded due to a "
+        "resolved collision", () {
+      initGraph({
+        "app|foo.a": "foo.a",
+        "app|foo.x": "foo.x"
+      }, {"app": [
+        [new TransformerGroup([[
+          new CheckContentAndRenameTransformer(
+              "a", "new foo.a", "z", "modified foo.a"),
+          new RewriteTransformer('x', 'a')
+        ]])]
+      ]});
+
+      updateSources(["app|foo.a", "app|foo.x"]);
+      expectAsset("app|foo.a", "foo.a");
+      expectNoAsset("app|foo.z");
+      buildShouldFail([isAssetCollisionException("app|foo.a")]);
+
+      modifyAsset('app|foo.a', 'new foo.a');
+      updateSources(["app|foo.a"]);
+      expectAsset("app|foo.a", "foo.x.a");
+      expectAsset("app|foo.z", "modified foo.a");
+      buildShouldSucceed();
+    });
   });
 
   test("runs transforms in an added group", () {
@@ -399,3 +426,27 @@ main() {
     buildShouldFail([isAssetCollisionException("app|foo.c")]);
   });
 }
+
+/// 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 {

[Bob Nystrom, 2013/10/08 21:07:36]

Move this to its own file in test/transformer.

[nweiz, 2013/10/08 22:20:12]

Done. I didn't do that originally because it seemed so specific to the test that
uses it here, but sure.

+  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));
+    });
+  }
+}