Update pub to use the new command runner API in args.

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

Index: sdk/lib/_internal/pub/lib/src/command/downgrade.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/downgrade.dart b/sdk/lib/_internal/pub/lib/src/command/downgrade.dart
index d3313a49b104dfedef48800d70473cad762811f7..c48c1f01857274bbb5b58e531054a2e3652aec7e 100644
--- a/sdk/lib/_internal/pub/lib/src/command/downgrade.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/downgrade.dart
@@ -12,26 +12,26 @@ 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 usage => "pub downgrade [dependencies...]";
-  bool get takesArguments => true;
+  String get invocation => "pub downgrade [dependencies...]";
 
-  bool get isOffline => commandOptions['offline'];
+  bool get isOffline => argResults['offline'];
 
   DowngradeCommand() {
-    commandParser.addFlag('offline',
+    argParser.addFlag('offline',
         help: 'Use cached packages instead of accessing the network.');
 
-    commandParser.addFlag('dry-run', abbr: 'n', negatable: false,
+    argParser.addFlag('dry-run', abbr: 'n', negatable: false,
         help: "Report what dependencies would change but don't change any.");
   }
 
-  Future onRun() async {
-    var dryRun = commandOptions['dry-run'];
+  Future run() async {
+    var dryRun = argResults['dry-run'];
     await entrypoint.acquireDependencies(SolveType.DOWNGRADE,
-        useLatest: commandOptions.rest, dryRun: dryRun);
+        useLatest: argResults.rest, dryRun: dryRun);
     if (isOffline) {
       log.warning("Warning: Downgrading when offline may not update you to "
           "the oldest versions of your dependencies.");