Remove barback from the repo.

pkg/barback/example/simple_transformer/lib/transformer.dart

-// Copyright (c) 2012, 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.
-
-import 'package:barback/barback.dart';
-
-import 'dart:async';
-
-class InsertCopyright extends Transformer {
-  String copyright = "Copyright (c) 2014, the Example project authors.\n";
-
-  // A constructor named "asPlugin" is required. It can be empty, but
-  // it must be present. It is how pub determines that you want this
-  // class to be publicly available as a loadable transformer plugin.
-  InsertCopyright.asPlugin();
-
-  Future<bool> isPrimary(AssetId id) {
-    return new Future.value(id.extension == '.txt');
-  }
-
-  Future apply(Transform transform) {
-    return transform.primaryInput.readAsString().then((content) {
-      var id = transform.primaryInput.id;
-      String newContent = copyright + content;
-      transform.addOutput(new Asset.fromString(id, newContent));
-    });
-  }
-}