Fix a bug in barback caused by r33408.

pkg/barback/test/package_graph/group_test.dart

Index: pkg/barback/test/package_graph/group_test.dart
diff --git a/pkg/barback/test/package_graph/group_test.dart b/pkg/barback/test/package_graph/group_test.dart
index 2f9fa4064d9a4ed8e2dd65b20e5d88bf8bf441d4..2588ebfc5570069d6a437de5da6a234aca92228d 100644
--- a/pkg/barback/test/package_graph/group_test.dart
+++ b/pkg/barback/test/package_graph/group_test.dart
@@ -366,6 +366,21 @@ main() {
     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");