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

sdk/lib/_internal/pub_generated/lib/src/command/downgrade.dart

Index: sdk/lib/_internal/pub_generated/lib/src/command/downgrade.dart
diff --git a/sdk/lib/_internal/pub_generated/lib/src/command/downgrade.dart b/sdk/lib/_internal/pub_generated/lib/src/command/downgrade.dart
deleted file mode 100644
index 87364022c3866314c03ccc690a74e75a7f95a49a..0000000000000000000000000000000000000000
--- a/sdk/lib/_internal/pub_generated/lib/src/command/downgrade.dart
+++ /dev/null
@@ -1,68 +0,0 @@
-// 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.
-
-library pub.command.downgrade;
-
-import 'dart:async';
-
-import '../command.dart';
-import '../log.dart' as log;
-import '../solver/version_solver.dart';
-
-/// Handles the `downgrade` pub command.
-class DowngradeCommand extends PubCommand {
-  String get name => "downgrade";
-  String get description =>
-      "Downgrade the current package's dependencies to oldest versions.\n\n"
-          "This doesn't modify the lockfile, so it can be reset with \"pub get\".";
-  String get invocation => "pub downgrade [dependencies...]";
-
-  bool get isOffline => argResults['offline'];
-
-  DowngradeCommand() {
-    argParser.addFlag(
-        'offline',
-        help: 'Use cached packages instead of accessing the network.');
-
-    argParser.addFlag(
-        'dry-run',
-        abbr: 'n',
-        negatable: false,
-        help: "Report what dependencies would change but don't change any.");
-  }
-
-  Future run() {
-    final completer0 = new Completer();
-    scheduleMicrotask(() {
-      try {
-        var dryRun = argResults['dry-run'];
-        new Future.value(
-            entrypoint.acquireDependencies(
-                SolveType.DOWNGRADE,
-                useLatest: argResults.rest,
-                dryRun: dryRun)).then((x0) {
-          try {
-            x0;
-            join0() {
-              completer0.complete();
-            }
-            if (isOffline) {
-              log.warning(
-                  "Warning: Downgrading when offline may not update you to "
-                      "the oldest versions of your dependencies.");
-              join0();
-            } else {
-              join0();
-            }
-          } catch (e0, s0) {
-            completer0.completeError(e0, s0);
-          }
-        }, onError: completer0.completeError);
-      } catch (e, s) {
-        completer0.completeError(e, s);
-      }
-    });
-    return completer0.future;
-  }
-}