Fix a bug in barback caused by r33408.

pkg/barback/test/package_graph/group_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.group_test;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:scheduled_test/scheduled_test.dart';
@@ skipping 348 matching lines @@
       [group, new RewriteTransformer("a", "z")]
     ]);
     expectAsset("app|foo.c", "foo.b.c");
     expectAsset("app|foo.z", "foo.z");
     buildShouldSucceed();
 
     expect(rewrite1.numRuns, completion(equals(1)));
     expect(rewrite2.numRuns, completion(equals(1)));
   });
 
+  test("doesn't run transforms in a removed group", () {
+    var rewrite1 = new RewriteTransformer("a", "b");
+    var rewrite2 = new RewriteTransformer("b", "c");
+    var group = new TransformerGroup([[rewrite1], [rewrite2]]);
+    initGraph(["app|foo.a"], {"app": [[group]]});
+
+    updateSources(["app|foo.a"]);
+    expectAsset("app|foo.c", "foo.b.c");
+    buildShouldSucceed();
+
+    updateTransformers("app", []);
+    expectNoAsset("app|foo.c");
+    buildShouldSucceed();
+  });
+
   test("doesn't pass through an input that's overwritten by an added group",
       () {
     var rewrite = new RewriteTransformer("x", "z");
     initGraph(["app|foo.a"], {"app": [[rewrite]]});
 
     updateSources(["app|foo.a"]);
     expectAsset("app|foo.a", "foo");
     buildShouldSucceed();
 
     updateTransformers("app", [
@@ skipping 20 matching lines @@
       new TransformerGroup([
         [new RewriteTransformer("a", "b")],
         [new RewriteTransformer("b", "c")]
       ]),
       new RewriteTransformer("x", "c")
     ]]});
     updateSources(["app|foo.a", "app|foo.x"]);
     buildShouldFail([isAssetCollisionException("app|foo.c")]);
   });
 }