Remove barback from the repo.

pkg/barback/test/transformer/declaring_rewrite.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.
-
-library barback.test.transformer.declaring_rewrite;
-
-import 'package:barback/barback.dart';
-
-import 'rewrite.dart';
-
-/// Like [RewriteTransformer], but declares its assets ahead of time.
-class DeclaringRewriteTransformer extends RewriteTransformer
-    implements DeclaringTransformer {
-  DeclaringRewriteTransformer(String from, String to)
-      : super(from, to);
-
-  void declareOutputs(DeclaringTransform transform) {
-    if (consumePrimary) transform.consumePrimary();
-    for (var extension in to.split(" ")) {
-      var id = transform.primaryId.changeExtension(".$extension");
-      transform.declareOutput(id);
-    }
-  }
-}