Fix a subtle bug in barback.

pkg/barback/test/package_graph/add_remove_transform_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.transform_test;
 
 import 'package:barback/src/utils.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../utils.dart';
@@ skipping 234 matching lines @@
     });
 
     updateSources(["pkg1|foo.txt", "pkg2|foo.inc"]);
     expectAsset("pkg1|foo.out", "foo.inc.inc");
     buildShouldSucceed();
 
     updateTransformers("pkg2", [[rewrite]]);
     expectAsset("pkg1|foo.out", "foo.inc");
     buildShouldSucceed();
   });
+
+  // Regression test.
+  test("a phase is added, then an input is removed and re-added", () {
+    var rewrite = new RewriteTransformer("txt", "mid");
+    initGraph(["app|foo.txt"], {
+      "app": [[rewrite]]
+    });
+
+    updateSources(["app|foo.txt"]);
+    expectAsset("app|foo.mid", "foo.mid");
+    buildShouldSucceed();
+
+    updateTransformers("app", [
+      [rewrite],
+      [new RewriteTransformer("mid", "out")]
+    ]);
+    expectAsset("app|foo.out", "foo.mid.out");
+    buildShouldSucceed();
+
+    removeSources(["app|foo.txt"]);
+    expectNoAsset("app|foo.out");
+    buildShouldSucceed();
+
+    updateSources(["app|foo.txt"]);
+    expectAsset("app|foo.out", "foo.mid.out");
+    buildShouldSucceed();
+  });
 }