Convert more pub code to use async/await.

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;
@@ skipping 10 matching lines @@
   bool get isOffline => commandOptions['offline'];
 
   DowngradeCommand() {
     commandParser.addFlag('offline',
         help: 'Use cached packages instead of accessing the network.');
 
     commandParser.addFlag('dry-run', abbr: 'n', negatable: false,
         help: "Report what dependencies would change but don't change any.");
   }
 
-  Future onRun() {
+  Future onRun() async {
     var dryRun = commandOptions['dry-run'];
-    return entrypoint.acquireDependencies(SolveType.DOWNGRADE,
-        useLatest: commandOptions.rest, dryRun: dryRun).then((_) {
-      if (isOffline) {
-        log.warning("Warning: Downgrading when offline may not update you to "
-            "the oldest versions of your dependencies.");
-      }
-    });
+    await entrypoint.acquireDependencies(SolveType.DOWNGRADE,
+        useLatest: commandOptions.rest, dryRun: dryRun);
+    if (isOffline) {
+      log.warning("Warning: Downgrading when offline may not update you to "
+          "the oldest versions of your dependencies.");
+    }
   }
 }