Only run [Transformer.isPrimary] once for each asset/transformer pair.

pkg/barback/test/package_graph/transform/concurrency_test.dart

 // Copyright (c) 2014, 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.
 
 /// This library contains tests for transformer behavior that relates to actions
 /// happening concurrently or other complex asynchronous timing behavior.
 library barback.test.package_graph.transform.concurrency_test;
 
 import 'package:barback/src/utils.dart';
 import 'package:scheduled_test/scheduled_test.dart';
@@ skipping 472 matching lines @@
     ]});
 
     rewrite.pauseApply();
     updateSources(["app|foo.in", "app|bar.txt"]);
     expectAssetDoesNotComplete("app|bar.out");
 
     rewrite.resumeApply();
     expectAsset("app|bar.out", "foo.in");
     buildShouldSucceed();
   });
+
+  test("materializes a passed-through asset that was emitted before it was "
+      "available", () {
+    initGraph(["app|foo.in"], {"app": [
+      [new RewriteTransformer("txt", "txt")]
+    ]});
+
+    pauseProvider();
+    updateSources(["app|foo.in"]);
+    expectAssetDoesNotComplete("app|foo.in");
+
+    resumeProvider();
+    expectAsset("app|foo.in", "foo");
+    buildShouldSucceed();
+  });
 }