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

sdk/lib/_internal/pub_generated/asset/dart/serialize/get_input_transform.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 pub.asset.serialize.get_input_transform;
-
-import 'dart:async';
-import 'dart:convert';
-
-import 'package:barback/barback.dart';
-
-import '../utils.dart';
-
-/// A mixin for transforms that support [getInput] and the associated suite of
-/// methods.
-abstract class GetInputTransform {
-  Future<Asset> getInput(AssetId id);
-
-  Future<String> readInputAsString(AssetId id, {Encoding encoding}) {
-    if (encoding == null) encoding = UTF8;
-    return getInput(id).then((input) =>
-        input.readAsString(encoding: encoding));
-  }
-
-  Stream<List<int>> readInput(AssetId id) =>
-      futureStream(getInput(id).then((input) => input.read()));
-
-  Future<bool> hasInput(AssetId id) {
-    return getInput(id).then((_) => true).catchError((error) {
-      if (error is AssetNotFoundException && error.id == id) return false;
-      throw error;
-    });
-  }
-}