Re-run a transform when a secondary input starts existing.

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 'package:barback/src/utils.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../utils.dart';
@@ skipping 158 matching lines @@
 
   test("a transformer can catch an error loading a secondary input", () {
     initGraph(["app|foo.txt"], {"app": [
       [new CatchAssetNotFoundTransformer(".txt", "app|nothing")]
     ]});
 
     updateSources(["app|foo.txt"]);
     expectAsset("app|foo.txt", "failed to load app|nothing");
     buildShouldSucceed();
   });
+
+  test("a transformer that fails due to a missing secondary input is re-run "
+      "when that input appears", () {
+    initGraph({
+      "app|foo.txt": "bar.inc",
+      "app|bar.inc": "bar"
+    }, {"app": [
+      [new ManyToOneTransformer("txt")]
+    ]});
+
+    updateSources(["app|foo.txt"]);
+    expectNoAsset("app|foo.out");
+    buildShouldFail([isMissingInputException("app|bar.inc")]);
+
+    updateSources(["app|bar.inc"]);
+    expectAsset("app|foo.out", "bar");
+    buildShouldSucceed();
+  });
 }