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
new file mode 100644
index 0000000000000000000000000000000000000000..87364022c3866314c03ccc690a74e75a7f95a49a
--- /dev/null
+++ b/sdk/lib/_internal/pub_generated/lib/src/command/downgrade.dart
@@ -0,0 +1,68 @@
+// 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;
+  }
+}