Support passing barback assets through phases in which they're unused.

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 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:barback/src/utils.dart';
@@ skipping 266 matching lines @@
     // next version of foo.out in line.
     // TODO(nweiz): Should this emit a collision error as well? Or should they
     // only be emitted when a file is added or removed?
     updateSources(["app|foo.three"]);
     buildShouldSucceed();
 
     removeSources(["app|foo.one"]);
     expectAsset("app|foo.out", "two.out");
     buildShouldFail([isAssetCollisionException("app|foo.out")]);
   });
+
+  test("a collision with a pass-through asset returns the pass-through asset",
+      () {
+    initGraph([
+      "app|foo.txt",
+      "app|foo.in"
+    ], {"app": [
+      [new RewriteTransformer("in", "txt")]
+    ]});
+
+    updateSources(["app|foo.txt", "app|foo.in"]);
+    expectAsset("app|foo.txt", "foo");
+    buildShouldFail([isAssetCollisionException("app|foo.txt")]);
+  });
+
+  test("a new pass-through asset that collides returns the previous asset", () {
+    initGraph([
+      "app|foo.txt",
+      "app|foo.in"
+    ], {"app": [
+      [new RewriteTransformer("in", "txt")]
+    ]});
+
+    updateSources(["app|foo.in"]);
+    expectAsset("app|foo.txt", "foo.txt");
+    buildShouldSucceed();
+
+    updateSources(["app|foo.txt"]);
+    expectAsset("app|foo.txt", "foo.txt");
+    buildShouldFail([isAssetCollisionException("app|foo.txt")]);
+  });
+
+  test("a new transform output that collides with a pass-through asset returns "
+      "the pass-through asset", () {
+    initGraph([
+      "app|foo.txt",
+      "app|foo.in"
+    ], {"app": [
+      [new RewriteTransformer("in", "txt")]
+    ]});
+
+    updateSources(["app|foo.txt"]);
+    expectAsset("app|foo.txt", "foo");
+    buildShouldSucceed();
+
+    updateSources(["app|foo.in"]);
+    expectAsset("app|foo.txt", "foo");
+    buildShouldFail([isAssetCollisionException("app|foo.txt")]);
+  });
 }