Remove barback from the repo.

pkg/barback/test/transformer/emit_nothing.dart

Index: pkg/barback/test/transformer/emit_nothing.dart
diff --git a/pkg/barback/test/transformer/emit_nothing.dart b/pkg/barback/test/transformer/emit_nothing.dart
deleted file mode 100644
index 9cf513446ea228c15ef205fb32abd188d2749dfa..0000000000000000000000000000000000000000
--- a/pkg/barback/test/transformer/emit_nothing.dart
+++ /dev/null
@@ -1,24 +0,0 @@
-// 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.
-
-library barback.test.transformer.emit_nothing;
-
-import 'package:barback/barback.dart';
-
-import 'mock.dart';
-
-/// A transformer that emits no assets.
-class EmitNothingTransformer extends MockTransformer {
-  final String extension;
-
-  EmitNothingTransformer(this.extension);
-
-  bool doIsPrimary(AssetId id) => id.extension == ".$extension";
-
-  void doApply(Transform transform) {
-    // Emit nothing.
-  }
-
-  String toString() => "$extension->nothing";
-}