Remove barback from the repo.

pkg/barback/test/transformer/sync_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.sync_rewrite;
-
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-/// Like [DeclaringRewriteTransformer], but with no methods returning Futures.
-class SyncRewriteTransformer extends Transformer
-    implements DeclaringTransformer {
-  final String from;
-  final String to;
-
-  SyncRewriteTransformer(this.from, this.to);
-
-  bool isPrimary(AssetId id) => id.extension == ".$from";
-
-  void apply(Transform transform) {
-    for (var extension in to.split(" ")) {
-      var id = transform.primaryInput.id.changeExtension(".$extension");
-      transform.addOutput(new Asset.fromString(id, "new.$extension"));
-    }
-  }
-
-  void declareOutputs(DeclaringTransform transform) {
-    for (var extension in to.split(" ")) {
-      var id = transform.primaryId.changeExtension(".$extension");
-      transform.declareOutput(id);
-    }
-  }
-
-  String toString() => "$from->$to";
-}