Enable cross-package transforms to see new transformers in other packages.

pkg/barback/test/package_graph/add_remove_transform_test.dart

Index: pkg/barback/test/package_graph/add_remove_transform_test.dart
diff --git a/pkg/barback/test/package_graph/add_remove_transform_test.dart b/pkg/barback/test/package_graph/add_remove_transform_test.dart
index b79c69afed00b1b9db7121436a4eaf60166595f2..bd68f68b2c3acea7c8eef3538fcd6ff32fb5a423 100644
--- a/pkg/barback/test/package_graph/add_remove_transform_test.dart
+++ b/pkg/barback/test/package_graph/add_remove_transform_test.dart
@@ -191,9 +191,6 @@ main() {
   });
 
   test("a cross-package transform sees a new transformer in a new phase", () {
-    // TODO(nweiz): make this work.
-    return;
-
     var rewrite = new RewriteTransformer("inc", "inc");
     initGraph({
       "pkg1|foo.txt": "pkg2|foo.inc",
@@ -204,7 +201,7 @@ main() {
     });
 
     updateSources(["pkg1|foo.txt", "pkg2|foo.inc"]);
-    expectAsset("pkg1|foo.out", "foo");
+    expectAsset("pkg1|foo.out", "foo.inc");
     buildShouldSucceed();
 
     updateTransformers("pkg2", [