Start pulling pub from its own repo.

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

-// 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() async {
-    var dryRun = argResults['dry-run'];
-    await entrypoint.acquireDependencies(SolveType.DOWNGRADE,
-        useLatest: argResults.rest, dryRun: dryRun);
-    if (isOffline) {
-      log.warning("Warning: Downgrading when offline may not update you to "
-          "the oldest versions of your dependencies.");
-    }
-  }
-}