Make PhaseOutput emit a new AssetNode whenever it changes outputs.

pkg/barback/test/package_graph/group_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.group_test;
 
+import 'dart:async';
+
 import 'package:barback/barback.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../utils.dart';
 
 main() {
   initConfig();
   test("runs transforms in a group", () {
     initGraph(["app|foo.a"], {"app": [
       [new TransformerGroup([
@@ skipping 289 matching lines @@
     test("doesn't detect a collision for an input that's modified in-place by "
         "a group", () {
       initGraph(["app|foo.a"], {"app": [[
         new TransformerGroup([[new RewriteTransformer("a", "a")]]),
         new RewriteTransformer("x", "y")
       ]]});
       updateSources(["app|foo.a"]);
       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", () {
     var rewrite = new RewriteTransformer("a", "z");
     initGraph(["app|foo.a"], {"app": [[rewrite]]});
 
     updateSources(["app|foo.a"]);
     expectAsset("app|foo.z", "foo.z");
     buildShouldSucceed();
 
@@ skipping 66 matching lines @@
       new TransformerGroup([
         [new RewriteTransformer("a", "b")],
         [new RewriteTransformer("b", "c")]
       ]),
       new RewriteTransformer("x", "c")
     ]]});
     updateSources(["app|foo.a", "app|foo.x"]);
     buildShouldFail([isAssetCollisionException("app|foo.c")]);
   });
 }