Remove barback from the repo.

pkg/barback/test/transformer/rewrite.dart

-// Copyright (c) 2013, 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.rewrite;
-
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-import 'mock.dart';
-
-/// A [Transformer] that takes assets ending with one extension and generates
-/// assets with a given extension.
-///
-/// Appends the output extension to the contents of the input file.
-class RewriteTransformer extends MockTransformer {
-  final String from;
-  final String to;
-
-  /// Creates a transformer that rewrites assets whose extension is [from] to
-  /// one whose extension is [to].
-  ///
-  /// [to] may be a space-separated list in which case multiple outputs will be
-  /// created for each input.
-  RewriteTransformer(this.from, this.to);
-
-  bool doIsPrimary(AssetId id) => id.extension == ".$from";
-
-  Future doApply(Transform transform) {
-    return getPrimary(transform).then((input) {
-      return Future.wait(to.split(" ").map((extension) {
-        var id = input.id.changeExtension(".$extension");
-        return input.readAsString().then((content) {
-          transform.addOutput(new Asset.fromString(id, "$content.$extension"));
-        });
-      }));
-    });
-  }
-
-  String toString() => "$from->$to";
-}