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

pkg/barback/test/package_graph/transform/transform_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.transform_test;
 
 import 'package:barback/src/utils.dart';
 import 'package:scheduled_test/scheduled_test.dart';
@@ skipping 350 matching lines @@
 
       updateSources(['app|foo.txt', 'app|bar.txt']);
       expectAsset('app|foo.txt', 'app|bar.txt: true');
       buildShouldSucceed();
 
       removeSources(['app|bar.txt']);
       expectAsset('app|foo.txt', 'app|bar.txt: false');
       buildShouldSucceed();
     });
 
-    // TODO(nweiz): enable this when issue 17480 is fixed.
-    // test("re-runs the transformer when an input starts existing", () {
-    //   initGraph(["app|foo.txt", "app|bar.txt"], {'app': [
-    //     [new HasInputTransformer(['app|bar.txt'])]
-    //   ]});
-    //
-    //   updateSources(['app|foo.txt']);
-    //   expectAsset('app|foo.txt', 'app|bar.txt: false');
-    //   buildShouldSucceed();
-    //
-    //   updateSources(['app|bar.txt']);
-    //   expectAsset('app|foo.txt', 'app|bar.txt: true');
-    //   buildShouldSucceed();
-    // });
+    test("re-runs the transformer when an input starts existing", () {
+      initGraph(["app|foo.txt", "app|bar.txt"], {'app': [
+        [new HasInputTransformer(['app|bar.txt'])]
+      ]});
+    
+      updateSources(['app|foo.txt']);
+      expectAsset('app|foo.txt', 'app|bar.txt: false');
+      buildShouldSucceed();
+    
+      updateSources(['app|bar.txt']);
+      expectAsset('app|foo.txt', 'app|bar.txt: true');
+      buildShouldSucceed();
+    });
   });
 }