Revert "Revert "Use native async/await support in pub.""

sdk/lib/_internal/pub_generated/lib/src/barback/dart_forwarding_transformer.dart

Index: sdk/lib/_internal/pub_generated/lib/src/barback/dart_forwarding_transformer.dart
diff --git a/sdk/lib/_internal/pub_generated/lib/src/barback/dart_forwarding_transformer.dart b/sdk/lib/_internal/pub_generated/lib/src/barback/dart_forwarding_transformer.dart
deleted file mode 100644
index d1129248f483490ba159cec7f9a9e429d43ae830..0000000000000000000000000000000000000000
--- a/sdk/lib/_internal/pub_generated/lib/src/barback/dart_forwarding_transformer.dart
+++ /dev/null
@@ -1,32 +0,0 @@
-// 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 pub.dart_forwarding_transformer;
-
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-import '../utils.dart';
-
-/// A single transformer that just forwards any ".dart" file as an output when
-/// not in release mode.
-///
-/// Since the [Dart2JSTransformer] consumes its inputs, this is used in
-/// parallel to make sure the original Dart file is still available for use by
-/// Dartium.
-class DartForwardingTransformer extends Transformer {
-  /// The mode that the transformer is running in.
-  final BarbackMode _mode;
-
-  DartForwardingTransformer(this._mode);
-
-  String get allowedExtensions => ".dart";
-
-  Future apply(Transform transform) {
-    return newFuture(() {
-      transform.addOutput(transform.primaryInput);
-    });
-  }
-}